Bluetooth: L2CAP: Fix use of uninitializer pointer

l2cap_br_conn_req_reply expects valid bt_l2cap_chan pointer but this
is achieved only on accept() callback. Use l2cap_br_send_conn_rsp
instead for rejecting cases where no channel was accepted. This also
makes success path being 'primary' function path ie erros all always
handled inside if() statements.

Jira: ZEP-1405

Change-Id: I890b4fcf029afce65eba4f2ebae0b1094feb007f
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>