net/usb/r815x: avoid to call mdio functions for runtime-suspended device

Don't replace the usb_control_msg() with usbnet_{read,write}_cmd()
which couldn't be called inside suspend/resume callback. Keep the
basic functions unlimited. Instead, using usb_autopm_get_interface()
and usb_autopm_put_interface() in r815x_mdio_{read,write}().

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>