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>
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