MIPS: ath79: Separate AR913x SoC specific WMAC setup code

The device registration code can be shared between the different SoCs, but
the required setup code varies Move AR913x specific setup code into a
separate function in order to make adding support for another SoCs easier.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3029/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>