hv: fix return type of hv_post_message()

This function returns negative error codes, but because the type is u16
they get truncated into positive numbers.  It doesn't look like the
callers care, but we should fix it anyway as a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>