mwl8k: don't complain about oversized beacons in FINALIZE_JOIN

The FINALIZE_JOIN firmware command only looks at the first couple of
fields in the beacon, and therefore it's not necessary to complain if
the beacon is longer than 128 bytes.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>