USB: Gadget Ethernet: Re-enable Jumbo frames.

Fixes: <b3e3893e> ("net: use core MTU range checking")
which patched only one of two functions used to setup the
USB Gadget Ethernet driver, causing a serious performance
regression in the ability to increase mtu size above 1500.

Signed-off-by: John Greb <h3x4m3r0n@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>