staging: brcm80211: bugfix for div by zero in minstrel_ht_tx_status

Caused by brcmsmac.ko suppling a 0 value to Mac80211. Mac80211 subsequently
divides by this number. Bug only occurred on AMPDU traffic. This is a fix
for https://bugzilla.kernel.org/show_bug.cgi?id=32032

, titled
'Divide error in minstrel_ht_tx_status followed by hang', reported by
Wouter Cloetens.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>