ath9k: Prevent divide-by-zero upon bad beacon_interval.

A similar patch fixed crashes seen on an ath9k system
when testing against a broken ath10k AP.  This patch
is slightly less protective, but probably will do the
job and is less redundant.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>