80211: ifconfig replace MS() with _IEEE80211_MASKSHIFT()

As we did in the kernel in r366112 replace the MS() macro with the version(s)
added to the kernel: _IEEE80211_MASKSHIFT(). Also provide its counter part.
This will later allow use to use other macros defined in net80211 headers
here in ifconfig.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation