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: NMax Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/bitops.h

@@ -48,7 +48,7 @@ static inline int ffz(unsigned long x)
  * __ffs: Find first bit set in word. Return 0 for bit 0
  */
 
-static inline int __ffs(unsigned long x)
+static inline unsigned long __ffs(unsigned long x)
 {
 	return 31 - __cntlz(x & -x);
 }