usbnet: ipheth: update Kconfig description

This module has for a long time not been limited to iPhone <= 3GS.
Update description to match the actual state of the driver.

Remove dead link from 2010, instead reference an existing userspace
iOS device pairing implementation as part of libimobiledevice.

Signed-off-by: Foster Snowhill <forst@pen.gy>
Signed-off-by: David S. Miller <davem@davemloft.net>