powerpc/powernv: Fix spurious kerneldoc warnings in opal-prd.c

Comments opening with /** are parsed by kerneldoc and this causes the
following warning to be printed:

	arch/powerpc/platforms/powernv/opal-prd.c:31: warning: cannot understand
	function prototype: 'struct opal_prd_msg_queue_item '

opal_prd_mesg_queue_item is an internal data structure so there's no real
need for it to be documented at all. Fix up the comment to squash the
warning.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200804005410.146094-5-oohall@gmail.com