rt2x00: Remove HWMODE_{A,B,G}

rt2500usb initialized the SIFS and EIFS without using the
values coming from rt2x000lib. After this is fixed HWMODE_{A,B,G}
is now unused and can be removed in favour of the ieee80211_band
enumeration which could still be usefull later.

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