ath9k_hw: split the generic hardware code by hardware family

Move out the generic hardware family code out into their own
files, we have one for AR5008, AR9001, and AR9002 family (ar9002_hw.c)
and another file for the new AR9003 hardware family (ar9003_hw.c).

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