USB: g_multi: fix CONFIG_USB_G_MULTI_RNDIS usage

g_multi used CONFIG_USB_ETH_RNDIS to check if RNDIS option was requested
where it should check for CONFIG_USB_G_MULTI_RNDIS.  As a result, RNDIS
was never present in g_multi regardless of configuration.

This fixes changes made in commit 396cda90d228d0851f3d64c7c85a1ecf6b8ae1e8.

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