selftests: net: devlink_port_split.py: skip the test if no devlink device

When there is no devlink device, the following command will return:
  $ devlink -j dev show
  {dev:{}}

This will cause IndexError when trying to access the first element
in dev of this json dataset. Use the kselftest framework skip code
to skip this test in this case.

Example output with this change:
  # selftests: net: devlink_port_split.py
  # no devlink device was found, test skipped
  ok 7 selftests: net: devlink_port_split.py # SKIP

Link: https://bugs.launchpad.net/bugs/1928889


Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>