mac80211: Add new API for rate selection

This patch adds a new API for setting a TX rate mask in
drivers that have rate control in either the firmware or hardware.
This can be used for various purposes, for example, masking out the
11b rates in P2P operation.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>