net: ipheth: Add USB ID for iPad mini

Adds the USB device ID (0x12ab) to the ipheth network-over-USB-tethering
driver for iOS devices.  Applied and tested against mainline tag v3.10
(as well as 3.8.x and 3.6.y kernel for Raspbian on Raspberry pi)

Signed-off-by: Aaron Marburg <amarburg@notetofutureself.org>
Signed-off-by: David S. Miller <davem@davemloft.net>