ath9k: content DMA start / stop through the PCU lock

This helps align resets / RX enable & disable / TX stop / start.
Locking around the PCU is important to ensure the hardware doesn't
get stale data when working with DMA'able data.

This is part of a series of patches which fix stopping
TX DMA completley when requested on the driver.
For more details about this issue refer to this thread:

http://marc.info/?l=linux-wireless&m=128629803703756&w=2

Tested-by:  Ben Greear <greearb@candelatech.com>
Cc: Kyungwan Nam <kyungwan.nam@atheros.com>
Cc: stable@kernel.org
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>