ath6kl: propagate error values on ar6000_avail_ev()

When something fails we set up some generic error values,
instead keep the values from the callers and make sure to
pass them on.

Cc: Naveen Singh <nsingh@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>