net: reduce USB network driver config options.

USB network drivers are already handled in drivers/net/usb/Kconfig.
Let's save the maintenance burden of dependencies in drivers/net/Makefile.

The newly introduced USB_NET_DRIVERS umbrella config option defaults
to 'y' so as to minimize the changes of behavior.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>