Bluetooth: L2CAP: Process fixed channels in the RX thread

Now that the TX path and TX context (bt_conn_tx) has been redesigned
to free the contexts always in the system workqueue, it means the
system workqueue is the only context where their allocation may also
fail. This is particularly problematic with us having all L2CAP
channels (fixed & CoC alike) deferred to the system workqueue. It is
especially bad for fixed channels where being able to send responses
for SM, L2CAP signaling and ATT is critical to avoid timeouts for the
connection.

This patch moves the processing of all fixed L2CAP channels back to
the RX thread, thereby making it possible (and safe) to block while
waiting for a TX context to become available.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>