Provide MS() and SM() macros for 80211 and wireless drivers.

We have (two versions) of MS() and SM() macros which we use throughout
the wireless code.  Change all but three places (ath_hal, rtwn, and rsu)
to the newly provided _IEEE80211_MASKSHIFT() and _IEEE80211_SHIFTMASK()
macros.  Also change one internal case using both _S and _M instead of
just _S away from _M (one of the reasons rtwn and rsu were not changed).

This was done semi-mechanically.  No functional changes intended.