ath9k_hw: simplify rf attach and rename to ath9k_hw_rf_alloc_ext_banks()

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