uio_hv_generic: make ring buffer attribute for primary channel

The primary channel also needs a ring buffer attribute. This allows
application to check if kernel supports uio sub channels, and also
makes all channels use consistent API.

Fixes: 37b96a49 ("uio_hv_generic: support sub-channels")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>