brcm80211: fmac: discard packet received when net device not registered

A new feature in the dongle firmware requires a handshake during
firmware intialization. The request is sent in event packets which the
host driver is not able to handle before any net device registered.
Discard those packets as the context for handling it is missing.
The initialization handler will be added as part of feature support
code in the future.

Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>