staging: tidspbridge: use the right type for list_is_last

Removes the following warning:

  CC [M]  drivers/staging/tidspbridge/rmgr/rmm.o
drivers/staging/tidspbridge/rmgr/rmm.c: In function 'rmm_alloc':
drivers/staging/tidspbridge/rmgr/rmm.c:147: warning: passing
	argument 1 of 'list_is_last' from incompatible pointer type
include/linux/list.h:170: note: expected 'const struct list_head *'
	but argument is of type 'struct rmm_ovly_sect *'

Signed-off-by: Omar Ramirez Luna <omar.ramirez@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Ionut Nicu <ionut.nicu@mindbit.ro>