ath5k: Remove unused sc->curmode

sc->curmode is set but never used. Remove it and the helper function. Also the
ath5k_rate_update which is refered to in the comment does not exist (any more?)
so we don't need to setup the band in that place.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>