ath9k: optimize the usage of power save wrappers.

We need not wake up the chip even before mutex lock is acquired and also
that it is required only if we are going to drain the txq. So place the
wrappers accordingly and this change is also useful when there are no
pending frames in the txq as we do not wake up the chip unnecessarily.

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