uio_hv_generic: Fix another memory leak in error handling paths

Memory allocated by 'vmbus_alloc_ring()' at the beginning of the probe
function is never freed in the error handling path.

Add the missing 'vmbus_free_ring()' call.

Note that it is already freed in the .remove function.

Fixes: cdfa835c

 ("uio_hv_generic: defer opening vmbus until first use")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/0d86027b8eeed8e6360bc3d52bcdb328ff9bdca1.1620544055.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>