ath9k: propagate errors on ath_init_device() and request_irq()

We've cleaned up ath_init_device() and its children enough
to pass meaninful errors back from probe. When this fails
it means our device could not be initialized and a meaninful
error will have been passed.

Do the same for request_irq() and also synchronize the error
messages while at it.

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