spi: spi-geni-qcom: Calculate FIFO size as per word length

Transfer data gets packed as per the bits per word
parameter passed by SPI client.
So, consider bits per word parameter while calculating
FIFO size.

Change-Id: I08986d3b63e00c828ee0610fdaa6176159a4bb33
Signed-off-by: Dilip Kota <dkota@codeaurora.org>