MIPS: ath79: Remove 'ar913x' from common variable and function names

The wireless MAC specific variables and the registration code can be shared
between multiple SoCs. Remove the 'ar913x' part from the function and
variable names to avoid confusions.

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/3028/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>