staging: r8712u: Fix possible out-of-bounds index with TKIP and AES keys

Array XGrpKey has only 2 elements and uses (keyid - 1) as the index, which
allows the possibility of memory corruption from an out-of-bounds index.

This problem was reported by a new version of smatch.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>