IB/qib: Fix clang confusion of NULL pointer comparison

clang becomes confused due to the comparison to NULL in a integer constant
expression context:

 >> drivers/infiniband/hw/qib/qib_sysfs.c:413:1: error: static_assert expression is not an integral constant expression
    QIB_DIAGC_ATTR(rc_resends);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~
    drivers/infiniband/hw/qib/qib_sysfs.c:406:16: note: expanded from macro 'QIB_DIAGC_ATTR'
            static_assert(&((struct qib_ibport *)0)->rvp.n_##N != (u64 *)NULL);    \

Nathan found __same_type that solves this problem nicely, so use it instead.

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>