spi: cadence-qspi: Do not calibrate when device tree sets read delay

If the device tree provides a read delay value, use that directly and do
not perform the calibration procedure.

This allows the device tree to over-ride the read delay value in cases
where the read delay value obtained via calibration is incorrect. One
such example is the Cypress Semper flash. It needs a read delay of 4 in
octal DTR mode. But since the calibration procedure is run before the
flash is switched in octal DTR mode, it yields a read delay of 2. A
value of 4 works for both octal DTR and legacy modes.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>