spi: zynq_qspi: Correct baud rate divisor calculation

spi: zynq_qspi: Correct baud rate divisor calculation

This patch corrects the baud rate divisor calculation by
checking with max baud rate value and assigning default
baud rate value if it exceeds max value.
This fixes the issue of qspi flash detection as writing
the overflown value results in setting unintended bits
in the register.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>