spi: cadence-quadspi: Set dummy cycles from STIG commands

spi: cadence-quadspi: Set dummy cycles from STIG commands

If a command does not have an address phase it goes via the STIG path.
The dummy cycles are not initialized for the STIG commands. As a result,
STIG commands with dummy cycles will not work.

Initialize the dummy cycle field before issuing the STIG command to make
sure it is sent correctly. Move the code to calculate dummy cycle value
to a separate function so it is not repeated twice. DTR support will add
some more logic here to it is worth it to extract it out in a function.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20201222184425.7028-4-p.yadav@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>