staging: fsl-mc: abstract test for existence of fsl-mc bus

Add function to test for existence of an fsl-mc bus instance
instead of doing this by looking directly at a field in the
bus type struct.

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