staging: fsl-mc: abstract test for whether a dprc is a root dprc

Instead of relying on assumptions about fields in data
structures, abstract the test for whether a dprc is a root
dprc into a function.

Signed-off-by: Itai Katz <itai.katz@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>