staging: mt7621-mmc: Refactor suspend, resume

Refactor msdc_drv_{suspend, resume} by adding a new function
msdc_drv_pm, that cleans up the code and removes double code.

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