staging: fsl-mc: add missing fsl_mc comment in struct msi_desc

The mc-bus specific field, fsl_mc in struct msi_desc is missing its
comment so add it.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>