net: axienet: remove stale comment of axienet_open

axienet_open no longer return -ENODEV when PHY cannot be connected to
since commit d7cc3163 ("net: axienet: Support phy-less mode of operation")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>