spi: cadence_qspi_apb: Ensure baudrate doesn't exceed max value

spi: cadence_qspi_apb: Ensure baudrate doesn't exceed max value

Ensuring the baudrate divisor value doesn't exceed the max value
in the calculation.It will be capped at max value to ensure the
correct value being written into the register.

Example of the existing bug is when calculated div = 16. After and
with the mask, the value written to register is actually 0 (register
field for baudrate divisor). With this fix, the value written is now
15 which is max value for baudrate divisor.

Signed-off-by: Chin Liang See <clsee@altera.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jteki@openedev.com>
Cc: Dinh Nguyen <dinguyen@altera.com>