header, and I also added the ieee80211_is_cck_rate counterpart.

Various drivers currently create there own version of these functions,
but I guess the ieee80211 stack is the best place to provide such
routines.
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: James Ketrenos <jketreno@linux.intel.com>