spi: cadence_qspi: support FIFO width other than 4 bytes

This patch makes the code compatible with FIFO depths other than 4
bytes. It also simplify read/write FIFO loops.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagannadh Teki <jteki@openedev.com>