Drivers: hv: vmbus: Return -EINVAL for the sys files for unopened channels

Before 98f4c651, we returned zeros for unopened channels.
With 98f4c651, we started to return random on-stack values.

We'd better return -EINVAL instead.

Fixes: 98f4c651 ("hv: move ringbuffer bus attributes to dev_groups")
Cc: stable@vger.kernel.org
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>