spi: imx: fix the burst length at DMA mode and CPU mode

For DMA mode, the bus width of the DMA is equal to the size of data
word, so burst length should be configured as bits per word.

For CPU mode, because of the spi transfer len is in byte, so calculate
the total number of words according to spi transfer len and bits per
word, burst length should be configured as total data bits.

Signed-off-by: Carlos Song <carlos.song@nxp.com>
Reviewed-by: Clark Wang <xiaoning.wang@nxp.com>
Fixes: e9b220ae ("spi: spi-imx: correctly configure burst length when using dma")
Fixes: 5f66db08 ("spi: imx: Take in account bits per word instead of assuming 8-bits")
Link: https://lore.kernel.org/r/20240204091912.36488-1-carlos.song@nxp.com


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