spi: bcm2835: fix overflow in calculation of transfer time

This resulted in the use of polling mode when other approaches
(dma or interrupts) would have been more appropriate.

Happened for transfers longer than 477 bytes.
Reported-by: Noralf Tronnes <noralf@tronnes.org>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -480,7 +480,7 @@ static int bcm2835_spi_transfer_one_poll(struct spi_master *master,
 					 struct spi_device *spi,
 					 struct spi_transfer *tfr,
 					 u32 cs,
-					 unsigned long xfer_time_us)
+					 unsigned long long xfer_time_us)
 {
 	struct bcm2835_spi *bs = spi_master_get_devdata(master);
 	unsigned long timeout;
@@ -531,7 +531,8 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
 {
 	struct bcm2835_spi *bs = spi_master_get_devdata(master);
 	unsigned long spi_hz, clk_hz, cdiv;
-	unsigned long spi_used_hz, xfer_time_us;
+	unsigned long spi_used_hz;
+	unsigned long long xfer_time_us;
 	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
 
 	/* set clock */
@@ -573,9 +574,10 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
 	bs->rx_len = tfr->len;
 
 	/* calculate the estimated time in us the transfer runs */
-	xfer_time_us = tfr->len
+	xfer_time_us = (unsigned long long)tfr->len
 		* 9 /* clocks/byte - SPI-HW waits 1 clock after each byte */
-		* 1000000 / spi_used_hz;
+		* 1000000;
+	do_div(xfer_time_us, spi_used_hz);
 
 	/* for short requests run polling*/
 	if (xfer_time_us <= BCM2835_SPI_POLLING_LIMIT_US)