xtensa: fix __ffs result type

Make __ffs result type unsigned long to match generic asm
implementation. This fixes the following build warning:

  mm/nobootmem.c: In function '__free_pages_memory':
  include/linux/kernel.h:742:17: warning: comparison of distinct pointer
                                          types lacks a cast
    (void) (&_min1 == &_min2);  \
  		 ^
  mm/nobootmem.c:100:11: note: in expansion of macro 'min'
     order = min(MAX_ORDER - 1UL, __ffs(start));

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>