drivers/net/usb: Add new driver ipheth

Add new driver to use tethering with an iPhone device. After initial submission,
apply fixes to fit the new driver into the kernel standards.

There are still a couple of minor (almost cosmetic-level) issues, but the driver
is fully functional right now.

Signed-off-by: L. Alberto Giménez <agimenez@sysvalve.es>
Signed-off-by: Diego Giagio <diego@giagio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>