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>

include/linux/msi.h

@@ -66,6 +66,7 @@ struct fsl_mc_msi_desc {
  * @mask_pos:	[PCI MSI]   Mask register position
  * @mask_base:	[PCI MSI-X] Mask register base address
  * @platform:	[platform]  Platform device specific msi descriptor data
+ * @fsl_mc:	[fsl-mc]    FSL MC device specific msi descriptor data
  */
 struct msi_desc {
 	/* Shared device/bus type independent data */