spi: cadence-qspi: Fix programming ospi flash speed

spi: cadence-qspi: Fix programming ospi flash speed

When the requested flash speed is 0, the baudrate division for the
requested speed causing drop in the performance. So set the ospi flash
to operate at max frequency when requested speed is zero.

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/20220512100535.16364-6-ashok.reddy.soma@xilinx.com
Signed-off-by: Michal Simek <michal.simek@amd.com>