cfg80211: fix and clean up cfg80211_gen_new_bssid()

Fix cfg80211_gen_new_bssid() to not rely on u64 modulo arithmetic,
which isn't needed since we really just want to mask there. Also,
clean it up to calculate the mask only once and use GENMASK_ULL()
instead of open-coding the mask calculation.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>