spi: omap2-mcspi: Move bytes per word calculation to the function

Introduce mcspi_bytes_per_word function as replacement for the next code
fragment:

int c = (word_len <= 8)  ? 1 :
	(word_len <= 16) ? 2 :
	/* word_len <= 32 */ 4;

This code used 2 times in current driver code and will be used 2 times in
the next FIFO buffer support patch. Replace it with inline function with clear
name to improve code legibility.

Signed-off-by: Illia Smyrnov <illia.smyrnov@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>