ncsid: update_ra_gw: Fix old_rtr check

An iteration of a previous commit changed the default value of `old_rtr`
from empty string to explicitly `invalid`. We need to use the correct
comparison for early router setting.

Change-Id: I27c198a93cf06622c6fc6f01092f12d72e05f722
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed