Bluetooth: ATT: Avoid warning when no ATT request is pending

Avoid warning in att_handle_rsp, only call this when there is a
pending ATT request. This avoids a warning happens when elevating
security fails but higher security had not been requested by an
ATT request.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>

squash! Bluetooth: ATT: Remove warning when no ATT process pending