spi: ar934x: fix transfer size

If bits_per_word is configured, transfer only word amount
of data per iteration.

Signed-off-by: Oskari Lemmela <oskari@lemmela.net>
Link: https://lore.kernel.org/r/20211222055958.1383233-2-oskari@lemmela.net


Signed-off-by: Mark Brown <broonie@kernel.org>