mac80211: remove ieee80211_tx_rate dependency in rate mask code

Remove ieee80211_tx_rate dependency in rate_idx_match_legacy_mask(),
rate_idx_match_mcs_mask() and rate_idx_match_mask() in order to use the
previous logic to define a ratemask in rate_control_set_rates() for
station rate table. Moreover move rate mask definition logic in
rate_control_cap_mask()

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>