Bluetooth: Host: Handle req alloc failure in bt_gatt_unsubscribe
Bluetooth: Host: Handle req alloc failure in bt_gatt_unsubscribe

`gatt_write_ccc` may fail if it is unable to allocate a `bt_att_req`.
This change adds handling of this case to `bt_gatt_unsubscribe`.

Signed-off-by: Aleksander Wasaznik <c0d989573cb73e7c43339e7aaab9bc0cb8ebe8c5@nordicsemi.no>

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