scripts: west_commands: linkserver: fix probe serial number support

With commit f419ea79 (runner: linkerserver : remove the probe ID
hardcode), support was added to use serial numbers with the linkserver
--probe argument. However, one invocation of the argument was missed,
and still used the "probe index" syntax. Resolve this issue.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>