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.

Change-Id: I085ee8206f36a161a00b4968b1875e47f7c72fc1
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: 5d4071abd9a1e2b417beef31aaf7d45999e4882e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>