Drivers: hv: vmbus: Move __vmbus_open()

Pure function movement, no functional changes. The move is made, because
in a later change, __vmbus_open() will rely on some static functions
afterwards, so we separate the move and the modification of
__vmbus_open() in two patches to make it easy to review.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20200916034817.30282-3-boqun.feng@gmail.com


Signed-off-by: Wei Liu <wei.liu@kernel.org>