spi: dspi: clear SPI_SR before enable interrupt

Once dspi is used in uboot, the SPI_SR have been set by some value.
At this time, if kernel enable the interrupt before clear the
status flag, that will trigger the wrong interrupt.

Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>