rt2x00: Remove reset_tsf()

Specifications indicate the TSF registers are read-only,
so there is no point in writing 0 to those registers.

As far as I know there isn't another way to reset the
TSF registers. So removing these callbacks will notify
mac80211 about the lack of support.

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