mac80211: add check on hw->max_signal value on ieee80211_register_hw

mac80211: add check on hw->max_signal value on ieee80211_register_hw

When IEEE80211_HW_SIGNAL_UNSPEC is set, mac80211 will perform a
division by max_signal in ieee80211_bss_info_update. If max_signal
is not properly set by the driver (for example it is zero) this
leads to a divide error and crash.
Thanks to Larry Finger, who pointed me to this.
This patch adds in ieee80211_register_hw one more check to detect
this condition and eventually returns -EINVAL, as already done for
other checks already performed there.

Signed-off-by: andrea merello <andrea.merello@gmail.com>
[move to an already existing SIGNAL_UNSPEC check]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>