Bluetooth: GATT: Fix possible crash while resubscribing
Bluetooth: GATT: Fix possible crash while resubscribing

When reconnecting the code will attempt to recover the subscriptions
but it was not setting any callback causing the bt_att_req.func to be
NULL.

Fixes #5982

Signed-off-by: Luiz Augusto von Dentz <8530c5ea66a1bdbc08f98bfcf183c8be901b5990@intel.com>

Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,explora26/zephyr,punitvara/zephyr,zephyriot/zephyr,mbolivar/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,zephyriot/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyriot/zephyr,kraj/zephyr,kraj/zephyr,explora26/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,Vudentz/zephyr,zephyriot/zephyr