r8152: fix the checking of the usb speed

When the usb speed of the RTL8152 is not high speed, the USB_DEV_STAT[2:1]
should be equal to [0 1]. That is, the STAT_SPEED_FULL should be equal
to 2.

There is a easy way to check the usb speed by the speed field of the
struct usb_device. Use it to replace the original metheod.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Spotted-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>