Bluetooth: Initialize new l2cap_chan structure members

Structure members used by ERTM or streaming mode need to be
initialized when an ERTM or streaming mode link is configured.  Some
duplicate code is also eliminated by moving in to the ERTM init
function.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>