staging/slicoss: Fix operation may be undefined warning

gcc complains about an undefined operation:
slicoss.c:1417:19: warning: operation on 'rspq->pageindex' may be
undefined [-Wsequence-point]

The intended operation was (probably) to retrieve the pageindex + 1 and let
it wrap around if it reaches the num_pages.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>