ath9k: move hw specific btcoex info to ath_hw

Since we now access it via the ath_hw declare the ath_hw pointer
at the header of some routines and se it. ath9k.h no longer needs to
access btcoex.h and to adjust for this move ath_btcoex_set_weight()
into btcoex.h and instead give main.c a helper for setting initial
values upon drv_start()

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>