ath9k: move hw code to its own module

hw code for Atheros 802.11n hardware is commmon between
different chipsets. This moves this code into a separate
module, the next expected user of this code will be
the ath9k_htc module.

The ath9k/ dir is now selected by ATH9K_HW, an option which
gets selected by either ath9k or ath9k_htc, but remains
invisible for user menuconfig configuration. If either
ath9k or ath9k_htc will be compiled into the kernel
ath9k_hw will also be compiled in.

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