mac80211: fix queue selection for packets injected via monitor interface

Commit 'mac80211: fix skb buffering issue' added an ->ndo_select_queue()
for monitor interfaces which can end up dereferencing ieee802_1d_to_ac[]
beyond the end of the array for injected data packets (as skb->priority
isn't guaranteed to be zero or within [0:7]), which then triggers the
WARN_ON in net/core/dev.c:dev_cap_txqueue().  Fix this by always setting
the priority to zero on injected data frames.

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