staging: mt7621-mmc: Remove redundant driver owner assignment

Remove the owner assignment form the platform driver as
platform_driver_register() already initializes the owner.

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