Bluetooth: ATT: Fix conn parameter to req cb in att_reset

The conn pointer is still valid / not reused at this time and can be
used further up the stack as an identifer. This simplifies the API of
ATT, and fixes callbacks in GATT that pass on this value directly since
their API does not allow conn to be NULL.

Fixes #41794

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