Bluetooth: host: clear ATT pending flag on failure to send

`ATT_PENDING_SENT` wasn't cleared when L2CAP reported an error when sending
the packet. This resulted in the channel being unusable for ever, since we
only clear that bit on a response (that will never be sent).

Found when setting `CONFIG_SYS_CLOCK_TICKS_PER_SEC=32768` in the
`notify_multiple` test. The kicker was the bug didn't manifest when EATT
wasn't enabled:
- we were queuing two unsubscribes back to back in the test
- on UATT we have to wait for a req-rsp pair before enqueuing a new one
- on UATT we only have one channel anyways

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>