alx: remove redundant D0 power state set

Pci_enable_device_mem() will set device power state to D0,
so it's no need to do it again in alx_probe().
Also remove redundant PM Cap find code, because pci core
has been saved the pci device pm cap value.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>