Bluetooth: ATT: Fix adding timeout for NULL request

In certain cases a response may happen even before number of complete
packets is generated by the controller:

[bt] [DBG] att_req_destroy: (0x0011cfe0) req 0x0011daa0
[bt] [DBG] att_process: (0x0011cfe0)
[bt] [DBG] att_req_sent: (0x0011d780) conn 0x0011d820 att 0x0011d9a0
 att->req 0x00000000

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