Bluetooth: Move recently-added ERTM header packing functions

Moving these functions simplifies future patches by eliminating
forward declarations, makes future patches easier to review, and
better preserves 'git blame' information.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>