Bluetooth: Add a structure to carry ERTM data in skb control blocks

Every field from ERTM control headers is now carried in the control
block so it only has to be parsed or generated once, and can be
efficiently accessed throughout the ERTM code.

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