MIPS: ath79: Rename dev-ar913x-wmac.c to dev-wmac.c

Rename the file as a last step of the 'ar913x' removal changes.

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/3034/


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