net/mlx5e: skip loopback selftest with !CONFIG_INET

When CONFIG_INET is disabled, the new selftest results in a link
error:

drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.o: In function `mlx5e_test_loopback':
en_selftest.c:(.text.mlx5e_test_loopback+0x2ec): undefined reference to `ip_send_check'
en_selftest.c:(.text.mlx5e_test_loopback+0x34c): undefined reference to `udp4_hwcsum'

This hides the specific test in that configuration.

Fixes: 0952da79

 ("net/mlx5e: Add support for loopback selftest")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>