GatewayScanController: test early for duration validity
GatewayScanController: test early for duration validity

Input arguments should be validated as soon as possible to refuse
invalid inputs early and avoid contention of the system.

Fixes: e3c58b4482cd ("GatewayScanController: new class")
Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server