Bluetooth: Update ERTM state before sending packets

Since l2cap_ertm_send() runs in process context, it may be preempted
by incoming data being processed in tasklet context.  In order to
avoid race conditions where a packet is acknowledged before the
transmit queue is properly updated, ERTM state should be set up for
the next transmit before the packet is actually sent.

CRs-fixed: 326238
Change-Id: I3df0e93118a357dbd34ff26ad74d7c0c38bf5121
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>