spi: zynq_qspi: Fix programming qspi speed

When programming qspi flash speed we need to check the requested flash
speed not to exceed the spi max frequency. In the current implementation
we are checking qspi ref clk instead. This commit fixes the issue by
checking the requested speed and programs the specified max frequency.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/1657893679-20039-5-git-send-email-ashok.reddy.soma@xilinx.com


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