uio_hv_generic: fix subchannel ring mmap

The fault method of handling subchannel ring, did not work correctly
(it only worked for the first page).

Since ring buffer is physically contiguous, using the vm helper
function is simpler and handles more cases.

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>