tests: net: getaddrinfo: Disable onboard Ethernet controller

If the tests/net/socket/getaddrinfo is run in a board that has
Ethernet controller, then it is possible that it will interfere
the test if the Ethernet cable is connected. As the test only
needs loopback support to run, disable the Ethernet as it is
not needed by the test.

Fixes #34923

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>