usb-gadget: fix warning in ethernet

Driver was taking max() of a size_t and u32 which causes complaint
about comparison of different types.

Stumbled on this accidently in my config, never used.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>