spi: zynq_qspi: Use dummy buswidth in dummy byte calculation

Fix dummy bytes calculation incase of valid dummy bytes when dummy
buswidth is > 1. Current dummy bytes calculation does not provide
correct dummy values for dummy buswidth > 1.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/1657893679-20039-3-git-send-email-ashok.reddy.soma@xilinx.com


Signed-off-by: Michal Simek <michal.simek@amd.com>