spi: fsl-dspi: Fix getting correct address for master

Current code set platform drvdata to dspi. However, the code in dspi_suspend()
and dspi_resume() assumes the drvdata is the address of master.
Fix it by setting platform drvdata to master.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org