MIPS: BCM47xx: Fix MAC address parsing.

Some devices like the Netgear WGT634u are using minuses between the blocks
of the MAC address and other devices are using colons to separate them.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2366/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>