docbook: fix rapidio warning

Fix fsl_rio.c kernel-doc warning: no exported symbols as
requested by !E are found:

  Warning(arch/powerpc/sysdev/fsl_rio.c): no structured comments found

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>