MIPS: BCM47xx: Use sscanf for parsing mac address

Instead of writing own function for parsing the mac address we now
use sscanf.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
To: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/1847/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>