staging: fsl-mc: Fix crash in fsl_mc_device_remove()

Only call fsl_mc_io_destroy() if the DPRC being removed
actually had an mc_io object associated with. Child DPRCs
that have not been bound to the DPRC driver or the VFIO driver
will not have an mc_io associated with them.

Signed-off-by: J. German Rivera <German.Rivera@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>