cris: intmem: fix pointer comparison compile warning

The code previously depended on list_head being defined
as the first item in struct intmem_allocation.

arch/cris/arch-v32/mm/intmem.c: In function ‘crisv32_intmem_free’:
arch/cris/arch-v32/mm/intmem.c:116:14: warning: comparison of distinct pointer types lacks a cast
    if ((prev != &intmem_allocations) &&
              ^
arch/cris/arch-v32/mm/intmem.c:123:14: warning: comparison of distinct pointer types lacks a cast
    if ((next != &intmem_allocations) &&
              ^

Signed-off-by: Niklas Cassel <nks@flawful.org>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>