ath9k: consolidate arguments on hw reset

HW reset calls pass the same variables or structs
which we can obtain easily from ah. Although this also applies
during channel changes as we will keep around the ath9k_channel
passed as an argument for now.

We now also now propagate the hw reset errors down.

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