Drivers: hv: vmbus: Add a helper function is_sub_channel()

The existing method of telling if a channel is sub-channel in
vmbus_process_offer() is cumbersome. This new simple helper function
is preferred in future.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>