spi: spi-fsl-dspi: Initialize completion before possible interrupt

The interrupt handler calls completion and is IRQ requested before the
completion is initialized.  Logically it should be the other way.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200622110543.5035-4-krzk@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>