MAINTAINERS: Add separate section for USB NETWORKING DRIVERS

There are several drivers in drivers/net/usb/ that
do not have specific MAINTAINERS that should have
emails forwarded to the linux-usb mailing list.

Add a section for those drivers.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>