staging: mt7621-mmc: Fix: copy_from_user() returns a positive value

copy_from_user() returns a positive value in case of an error, to fix
this the check is turned around, also a better return value is chosen.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>