ath9k_hw_rfattach() was just calling a helper and this helper was
doing nothing for single-chip devices, and for non single-chip devices
it is just allocating memory for banks to program the RF registers
at a later time. Simplify this by having the hw initialization call
the rf bank allocation directly for external radios.

Also, propagate an -ENOMEM properly now upon failure.
Signed-off-by: NLuis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>