spi: spi-fsl-dspi: Reduce indentation level in dspi_interrupt

If the entire function depends on the SPI status register having the
interrupt bits asserted, then just check it and exit early if those bits
aren't set (such as in the case of the shared IRQ being triggered for
the other peripheral). Cosmetic patch.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190822211514.19288-2-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>