UPSTREAM: 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>
(cherry picked from commit 5d4071abd9a1e2b417beef31aaf7d45999e4882e)
Bug: 154523213
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: Ia7f3782f7831b7994fde3cfae85308c6b73602d4