staging: mt7621-mmc: Fix error number assignment

Currently the value that data->error is set to is converted to an
unsigned int, but it is a usual error number, so it should be
negative.

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