mtd: m25p80: fix module autoloading for "jedec, spi-nor" and "spi-nor"

Commit 43163022 ("mtd: m25p80: allow arbitrary OF matching for
"jedec,spi-nor"") moved the "jedec,spi-nor" handling from the
spi_device_id table to the of_match_table, to better handle matching
complex device tree compatible strings. With that patch, device tree
support works as expected when m25p80.c is built into the kernel.

However, that commit ignored the fact that:

 (1) (non-DT) platform devices might want to use the "spi-nor" string
     for matching with this driver, rather than picking an arbitrary one
     like "m25p80"
 (2) the core SPI uevent/modalias code doesn't yet support kernel module
     autoloading via of_match_table strings; so for DT-based devices, it
     will only report (part of) the first compatible string used

Problem (1) has been reported previously, and I forgot to patch it up
afterward.

Problem (2) was noticed recently here:
http://lists.infradead.org/pipermail/linux-mtd/2015-October/062369.html
https://lkml.org/lkml/2015/11/12/574

Specifically, this patch fixes m25p80.ko module autoloading for cases
like this:

	flash@xxx {
		compatible = "jedec,spi-nor";
		...
	};

because modalias of "spi:spi-nor" (the only module loading info provided
by the SPI core for this device) will now be listed as an alias in
m25p80.ko.

Notably, it does *not* help cases like this:

	flash@xxx {
		compatible = "vendor,shiny-new-device", "jedec,spi-nor";
		...
	};

unless we also list "shiny-new-device" in m25p_ids[]. There has been
discussion on future work for this issue here:
https://lkml.org/lkml/2015/11/12/574

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>

drivers/mtd/devices/m25p80.c

@@ -163,7 +163,7 @@ static int m25p_probe(struct spi_device *spi)
 	struct m25p *flash;
 	struct spi_nor *nor;
 	enum read_mode mode = SPI_NOR_NORMAL;
-	char *flash_name = NULL;
+	char *flash_name;
 	int ret;
 
 	data = dev_get_platdata(&spi->dev);
@@ -202,6 +202,8 @@ static int m25p_probe(struct spi_device *spi)
 	 */
 	if (data && data->type)
 		flash_name = data->type;
+	else if (!strcmp(spi->modalias, "spi-nor"))
+		flash_name = NULL; /* auto-detect */
 	else
 		flash_name = spi->modalias;
 
@@ -235,6 +237,13 @@ static int m25p_remove(struct spi_device *spi)
  * keep them available as module aliases for existing platforms.
  */
 static const struct spi_device_id m25p_ids[] = {
+	/*
+	 * Allow non-DT platform devices to bind to the "spi-nor" modalias, and
+	 * hack around the fact that the SPI core does not provide uevent
+	 * matching for .of_match_table
+	 */
+	{"spi-nor"},
+
 	/*
 	 * Entries not used in DTs that should be safe to drop after replacing
 	 * them with "nor-jedec" in platform data.