ath9k: cleanup hw pll work handler

There is no reason why pll work handler should be part of xmit
file. move it to main.c so that reading hw check routines are
all in the same place.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>