Bluetooth: Check rules when setting retransmit or monitor timers

The ERTM specification requires the retransmit timer to be cancelled
when the monitor timer is set.  The retransmit timer cannot be set
again while the monitor timer is pending.

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