rt2x00: Forbid aggregation for STAs not programmed into the hw

If a STA is not known by the hw (i.e. has no WCID assigned) don't allow
aggregation since this might mess up tx status reports and we won't be
able to distinguish the reports of multiple WCID-less STAs.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>