staging: fsl-mc: fix device ref counting

Drop unneeded get_device() call at device creation
and, as per documentation, drop reference count
after using device_find_child() return.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Acked-by: Stuart Yoder <stuart.yoder@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>