USRP lib: fix incorrect config check for time_source fallback
Corrected a logic bug in usrp_lib.cpp where the code mistakenly checked openair0_cfg[0].clock_source instead of time_source when a time_source was already specified in usrp_args.
Also improved the warning log to clarify that the config value is being ignored in favor of the explicit user-provided usrp_args.
Co-authored-by: Alexandre Serio alexandre.j.serio@altice.pt