Bluetooth: L2CAP: Fix TX queueing for LE CoC

This fixes an issue found on automated testing. If IUT had no credits
to send data, the TX request was not queued, because -EAGAIN error was
not returned to bt_l2cap_chan_send (which was responsible for queueing).

Scenario:
1. IUT was out of credits
2. Tester requested to send some data
3. Credits was given
4. No data was sent

Fixes: ZEP-1896

Change-Id: Ie9d0945d1e6b628cd978ede8105b37b838a61f1a
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>