spi: imx: Use dynamic bursts only when bits_per_word is 8, 16 or 32

The dynamic bursts mode allows to group together multiple words into a
single burst. To do so, it's necessary that words can be packed into the
32-bits FIFO entries, so we can't allow using this mode with bit_per_words
different to 8, 16 or 32.

This prevents shitfing out extra clock ticks for transfers with
bit_per_word values not aligned on 8 bits.

With that , we are sure that only the correct number of bits is
shifted out at each transfer, so we don't need to mask out the remaining
parts of the words.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>