set errno to 0 in "switchtec_open_by_path" for inband
set errno to 0 in "switchtec_open_by_path" for inband

"switchtec_open_by_path" check fd by "isatty" first,
this api will set errno accordingly.
If fd is not tty, errno will be set to 25, this will
introduce false alarm for inband mode cmds, like:
fw-toggle & evcntr-setup.

Microsemi/switchtec-user,Microsemi/switchtec-user