drivers: usb: gadget: ether: access network_started using local variable

network_started of struct eth_dev can be accessed using local
variable dev and no reason to access it with the global struct.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>