spi: Introduce and use spi_bpw_to_bytes()

spi: Introduce and use spi_bpw_to_bytes()

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

Recently in the discussion with David the idea of having
a common helper popped up. The helper converts the given
bits per word to bytes. The result will always be power-of-two
(e.g. for 37 bits it returns 8 bytes) or 0 for 0 input.
More details are in the respective code comment.

This mini-series introduces it and replaces current users
under drivers/spi and we expect more (and possibly some
lurking in other subsystems).