Bluetooth: ATT: Fix overwritting sent callback
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 <8530c5ea66a1bdbc08f98bfcf183c8be901b5990@intel.com>

Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr