spi: loopback-test: mark rx_ranges_cmp() static

We get 1 warning when building kernel with W=1:
drivers/spi/spi-loopback-test.c:408:5: warning: no previous prototype for 'rx_ranges_cmp' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-loopback-test.c

@@ -405,7 +405,7 @@ struct rx_ranges {
 	u8 *end;
 };
 
-int rx_ranges_cmp(void *priv, struct list_head *a, struct list_head *b)
+static int rx_ranges_cmp(void *priv, struct list_head *a, struct list_head *b)
 {
 	struct rx_ranges *rx_a = list_entry(a, struct rx_ranges, list);
 	struct rx_ranges *rx_b = list_entry(b, struct rx_ranges, list);