spi: cadence-quadspi: Fix dummy cycle calculation when buswidth > 1

spi: cadence-quadspi: Fix dummy cycle calculation when buswidth > 1

SPI MEM deals with dummy bytes but the controller deals with dummy
cycles. Multiplying bytes by 8 is correct if the dummy phase uses 1S
mode since 1 byte will be sent in 8 cycles. But if the dummy phase uses
4S mode then 1 byte will be sent in 2 cycles.

To correctly translate dummy bytes to dummy cycles, the dummy buswidth
also needs to be taken into account. Divide 8 by the buswidth to get the
correct multiplier for getting the number of cycles.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20201222184425.7028-5-p.yadav@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>