Bluetooth: ATT: Fix overwritting sent callback

ATT channel sent callback shall not be overwritting until the
operation completes as it can result in breaking flow control when
CONFIG_BT_ATT_ENFORCE_FLOW is enabled.

Fixes #25964
Fixes #26071

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