ath9k_hw: order phy.c code and integrate spur mitigation

This reorders phy.c routines in the order in the order in which they are used
and also moves the spur mitigation helpers for each type of chip into phy.c
as they are RF related.

This patch has no functional changes.

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