drivers: spi: spi_max32: Fix word size support

Driver was not handling SPI word sizes other than 8 bits. Apply DFS
shift wherever necessary to support non 8-bit transfers.

DMA mode cannot support word sizes that are less than 8 bits so return
-ENOTSUP if word size less than 8-bits is required.

Signed-off-by: Tahsin Mutlugun <Tahsin.Mutlugun@analog.com>
(cherry picked from commit f47449e7)