Bluetooth: Skip ERTM queue processing when there are receive errors

An application may call recv() on a disconnected socket if the
connection has unexpectedly dropped.  In this case, the socket may
still exist, but it will be in an error state and the L2CAP connection
will be mostly torn down.

A crash was observed when the ERTM code attempted to send an ACK when
the L2CAP connection was not up.  If the error state is detected and
ERTM queue processing is skipped, then the crash is avoided and the
recv() call just returns an error in userspace.

CRs-fixed: 336000
Change-Id: I540d774c5d42f0df856b13b6e634e6ba31d86492
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>