spi: cadence_qspi: Fix baud rate calculation

spi: cadence_qspi: Fix baud rate calculation

With the existing code, when the requested SPI clock rate is near
to the lowest that can be achieved by the hardware (max divider
of the ref clock is 32), the generated clock rate is wrong.
For example, with a 50MHz ref clock, when asked for anything less
than a 1.5MHz SPI clock, the code sets up the divider to generate
25MHz.

This change fixes the calculation.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>