staging: brcm80211: remove brcmf_op_if from fullmac

Absorb related code into brcmf_add_if/brcmf_del_if. This is part
of the net device interface clean up.

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