rt2x00: Remove ieee80211_rx_status from rt2x00_dev

rt2x00 was keeping a copy of ieee80211_rx_status embedded
into the rt2x00_dev structure. For each RX frame, this structure
was copied into the skb->cb where mac80211 would handle it further.

However at the moment only the fields current band, and frequency
were updated. Whereas the band was already provided directly within
the rt2x00_dev structure. Save a memcpy action, and reduce memory
a bit, by adding a curr_freq field to rt2x00_dev, and completely
remove the ieee80211_rx_status structure from rt2x00_dev.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>