Bluetooth: ATT: Fix clearing context at disconnect

When att_disconnected is called a thread may be waiting for the tx_sem
but that is memset to 0, furthermore there exists a flag
ATT_DISCONNECTED to indicate the context is no longer valid so instead
move memset to bt_att_accept so it is cleared when it is about to be
reused.

Fixes #8083

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