Bluetooth: Make l2cap_clear_timer return if timer was running or not

This is usefull when need to make action after timer was cleared
depending on if it was running or not.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>