staging: mt7621-mmc: Replace sdr_write32 with writel

The current code uses a macro (sdr_write32) for writing to hardware,
but it is only a writel with switched arguments, so replace it to get
nearer to upstream code.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>