usbnet: smsc95xx: Clean up nopm handling

The LAN95xx driver has just been amended to auto-detect whether the
_nopm variant of usbnet_read_cmd() / usbnet_write_cmd() shall be used.

Drop all the now unnecessary open coding of that distinction.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: David S. Miller <davem@davemloft.net>