ath5k: remove double opmode definition

opmode (operating mode) was defined in struct ath5k_hw and struct ath5k_softc.
remove it from ath5k_hw and use only from ath5k_softc (sc->opmode).

(btw: what's the meaning of opmode when we have multiple interfaces?)

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