selftests: net: tsn_lib: don't overwrite isochron receiver extra args with UDS

The extra_args argument ($3) of isochron_recv_start is overwritten with
uds ($2), if that argument exists.

This is currently not a problem, because the only TSN selftest
(ocelot/psfp.sh) omits remote sync so it does not specify to the
receiver a UNIX domain socket for ptp4l. So $uds is currently an empty
string.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>