uio_hv_generic: set size of ring buffer attribute

The original code had ring size as a module parameter, but
then it was made a fixed value.  The code to set the size of
the ring buffer binary file was lost in the transistion.
The size is needed by user mode driver to know the size of
the ring buffer.

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>