csky: add missing brackets in a macro for tlb.h

As an earlier patch made the macro argument more complicated, compilation
now fails with:

 In file included from mm/madvise.c:30:
 mm/madvise.c: In function 'madvise_free_single_vma':
 arch/csky/include/asm/tlb.h:11:11: error:
     invalid type argument of '->' (have 'struct mmu_gather')

Link: https://lore.kernel.org/r/20190901193601.GB5208@mellanox.com


Fixes: 923bfc561e75 ("pagewalk: separate function pointers from iterator data")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>