ipheth: Add support for iPad 2 and iPad 3

Each iPad model has a different product id, this patch adds support for iPad 2
(pid 0x12a2) and iPad 3 (pid 0x12a6). Note that iPad 2 must be jailbroken and a
third-party app must be used for tethering to work. On iPad 3, tethering works
out of the box (assuming your ISP is nice).

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>