fsl-dspi: Correctly configure 'bits_per_word'

commit 34bded5b81465040bd2cec4666a6025af8b43581 from
https://source.codeaurora.org/external/autobsps32/linux



Issue: ALB-4840
Signed-off-by: Ciprian Marian Costea <ciprianmarian.costea@nxp.com>
[Xulin: adjusted the applying patch conflict.]
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>