staging: brcm80211: use mac_pton() instead of own implementation

The function brcmu_ether_atoe() does exactly the same as mac_pton().
The driver now uses the latter and brcmu_ether_atoe() has been removed
as it is not used anymore.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>