Unlock before returning on this error path.

Fixes: 111d746b ("misc: bcm-vk: add VK messaging support")
Acked-by: Desmond Yan <desmond.yan@broadcom.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YBfyb+jU5lDUe+5g@mwandaSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>