asix: AX88172A driver depends on phylib

Since commit 16626b0cc3d5afe250850f96759b241f8a403b52 the asix
driver depends on the phylib. Select phylib when the asix driver is
selected.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: kernel-janitors@vger.kernel.org
Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>