spi: Consistently use BIT for cs_index_mask

spi: Consistently use BIT for cs_index_mask

Some of the parts related to the chip select are using BIT() macro
the rest are using plain numbers. Unify all of them to use BIT().

While at it, make the (repetitive) comment clearer when assigning
cs_index_mask during SPI target device enumeration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://msgid.link/r/20240307150256.3789138-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>