USB: Added USB_ETH_RNDIS to use instead of CONFIG_USB_ETH_RNDIS

If g_ether and g_multi are both built CONFIG_USB_ETH_RNDIS symbol
may be redefined in the later and, whats even worse, g_ether's settings
may affect g_multi's.  This adds a USB_ETH_RNDIS symbol defined at the
beginning of ether.c and multi.c according toproper KConfig settings.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>