staging: brcm80211: use linux native ethertype iso ETH_P_BRCM

The linux native ethertype ETH_P_LINK_CTL is matching ETH_P_BRCM so
lets use that definition and get rid of ETH_P_BRCM.

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