tools/mgmt-tester: Refactor setup_start_discovery function
tools/mgmt-tester: Refactor setup_start_discovery function

This patch removes handling hook register from this
function and its callback as this is no longer necessary for any stop
discovery tests.

This patch also adds setup_send_param and setup_send_len  parameters to
test data so it is easy to control start discovery command parameters.
It is useful for tests:
Stop Discovery - Invalid parameters 1
Stop Discovery - BR/EDR (Inquiry) Success 1

silent-snowman/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,ComputeCycles/bluez,mapfau/bluez,silent-snowman/bluez,pkarasev3/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,mapfau/bluez,pkarasev3/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez