Bluetooth: L2CAP: Reset channel status before destroy callback

Move reset of channel status from after the destroy callback since the
after the destroy callback the memory should be assumed to be released.
Instead clear the channel status when the channel is created in
l2cap_chan_add. This way we don't rely on the memory given being set to
the correct value.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>