[media] smiapp-pll: Constify limits argument to smiapp_pll_calculate()

The limits are input parameters and should not be modified by the
smiapp_pll_calculate() function. Make them const.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>