ath5k: Support synth-only channel change for AR2413/AR5413

ath5k: Support synth-only channel change for AR2413/AR5413

 * Add synth-only channel change for AR2413/5413. When we call
 ath5k_reset with a channel ath5k_hw_reset will first try to
 set channel on PHY while PHY is running instead of doing a normal
 full reset. To do this phy_init has to change to implement this
 functionality.

 * Clean up change_channel flag, what it really did was skip PCU
 registers when setting initvals. This is done because on reset
 PCU registers are not affected (except the registers we set
 in pcu init and -due to hw problems- TSF). Use a new skip_pcu
 flag that's not misleading instead. In the future we might use
 that to also skip PCU reset and save us the TSF etc problems
 (needs testing because standard practice is to reset everything).

 * Use fast channel change only when setting channel, and set skip_pcu
 to false only on init. When we reset the card due to DMA or PHY
 problems skip pcu but never do a fast channel change.

 Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>