spi: work around clang bug in SPI_BPW_RANGE_MASK()

Clang-8 evaluates both sides of a ?: expression to check for
valid arithmetic even in the side that is never taken. This
results in a build warning:

drivers/spi/spi-sh-msiof.c:1052:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
        .bits_per_word_mask = SPI_BPW_RANGE_MASK(8, 32),
                              ^~~~~~~~~~~~~~~~~~~~~~~~~

Change the implementation to use the GENMASK() macro that does
what we want here but does not have a problem with the shift
count overflow.

Link: https://bugs.llvm.org/show_bug.cgi?id=38789
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -444,8 +444,7 @@ struct spi_controller {
 	/* bitmask of supported bits_per_word for transfers */
 	u32			bits_per_word_mask;
 #define SPI_BPW_MASK(bits) BIT((bits) - 1)
-#define SPI_BIT_MASK(bits) (((bits) == 32) ? ~0U : (BIT(bits) - 1))
-#define SPI_BPW_RANGE_MASK(min, max) (SPI_BIT_MASK(max) - SPI_BIT_MASK(min - 1))
+#define SPI_BPW_RANGE_MASK(min, max) GENMASK((min) - 1, (max) - 1)
 
 	/* limits on transfer speed */
 	u32			min_speed_hz;