spi: introduce macros to set bits_per_word_mask

Introduce two macros to make setting up spi_master.bits_per_word_mask
easier, and avoid mistakes like writing BIT(n) instead of BIT(n - 1).

SPI_BPW_MASK is for a single supported value of bits_per_word_mask.

SPI_BPW_RANGE_MASK represents a contiguous set of bit lengths.
Signed-off-by: NStephen Warren <swarren@wwwdotorg.org>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/spi/spi.h

@@ -308,6 +308,8 @@ struct spi_master {
 
 	/* bitmask of supported bits_per_word for transfers */
 	u32			bits_per_word_mask;
+#define SPI_BPW_MASK(bits) BIT((bits) - 1)
+#define SPI_BPW_RANGE_MASK(min, max) ((BIT(max) - 1) - (BIT(min) - 1))
 
 	/* other constraints relevant to this driver */
 	u16			flags;