Bluetooth: ATT: Fix not processing pending requests

Since the TX semaphore is used for all types of PDUs a request may have
to be put on the request list while there is no pending request pending
which means no response will be generated to trigger att_process,
previously this condition was handled by setting the request as
currently pending and append its buffer to tx_queue but this is no
longer efficient since there could be more than one channel active the
code should try all of them before queueing back to request list.

To fix this the request list will now be processed each time a PDU has
been sent.

Fixes #26070

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>