spi: spi-fsl-qspi: Clear TDH bits in FLSHCR register

Later versions of the QSPI controller (e.g. in i.MX6UL/ULL and i.MX7)
seem to have an additional TDH setting in the FLSHCR register, that
needs to be set in accordance with the access mode that is used (DDR
or SDR).

Previous bootstages such as BootROM or bootloader might have used the
DDR mode to access the flash. As we currently only use SDR mode, we
need to make sure the TDH bits are cleared upon initialization.

Fixes: 84d04318

 ("spi: Add a driver for the Freescale/NXP QuadSPI controller")
Cc: <stable@vger.kernel.org>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Acked-by: Han Xu <han.xu@nxp.com>
Link: https://lore.kernel.org/r/20191007071933.26786-1-frieder.schrempf@kontron.de
Signed-off-by: Mark Brown <broonie@kernel.org>