Bluetooth: Make l2cap_ertm_data_rcv static

It is not used outside of l2cap_core.c. Also l2cap_ertm_data_rcv is
only used after it is defined so there is no need for forward declaration.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>