ARC: fix __ffs return value to avoid build warnings

|  CC      mm/nobootmem.o
|In file included from ./include/asm-generic/bug.h:18:0,
|                 from ./arch/arc/include/asm/bug.h:32,
|                 from ./include/linux/bug.h:5,
|                 from ./include/linux/mmdebug.h:5,
|                 from ./include/linux/gfp.h:5,
|                 from ./include/linux/slab.h:15,
|                 from mm/nobootmem.c:14:
|mm/nobootmem.c: In function '__free_pages_memory':
|./include/linux/kernel.h:845:29: warning: comparison of distinct pointer types lacks a cast
|   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
|                             ^
|./include/linux/kernel.h:859:4: note: in expansion of macro '__typecheck'
|   (__typecheck(x, y) && __no_side_effects(x, y))
|    ^~~~~~~~~~~
|./include/linux/kernel.h:869:24: note: in expansion of macro '__safe_cmp'
|  __builtin_choose_expr(__safe_cmp(x, y), \
|                        ^~~~~~~~~~
|./include/linux/kernel.h:878:19: note: in expansion of macro '__careful_cmp'
| #define min(x, y) __careful_cmp(x, y, <)
|                   ^~~~~~~~~~~~~
|mm/nobootmem.c:104:11: note: in expansion of macro 'min'
|   order = min(MAX_ORDER - 1UL, __ffs(start));

Change __ffs return value from 'int' to 'unsigned long' as it
is done in other implementations (like asm-generic, x86, etc...)
to avoid build-time warnings in places where type is strictly
checked.

As __ffs may return values in [0-31] interval changing return
type to unsigned is valid.
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/bitops.h

@@ -340,7 +340,7 @@ static inline __attribute__ ((const)) int __fls(unsigned long x)
 /*
  * __ffs: Similar to ffs, but zero based (0-31)
  */
-static inline __attribute__ ((const)) int __ffs(unsigned long word)
+static inline __attribute__ ((const)) unsigned long __ffs(unsigned long word)
 {
 	if (!word)
 		return word;
@@ -400,9 +400,9 @@ static inline __attribute__ ((const)) int ffs(unsigned long x)
 /*
  * __ffs: Similar to ffs, but zero based (0-31)
  */
-static inline __attribute__ ((const)) int __ffs(unsigned long x)
+static inline __attribute__ ((const)) unsigned long __ffs(unsigned long x)
 {
-	int n;
+	unsigned long n;
 
 	asm volatile(
 	"	ffs.f	%0, %1		\n"  /* 0:31; 31(Z) if src 0 */