staging: rtl8723au: Eliminate set_wep_key() and call set_group_key() directly

This also eliminates another round of determine algorithm based on key
length, followed by determine key length based on algorithm.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>