9pfs: fix P9_NOTAG and P9_NOFID macros

The u16 and u32 types don't exist in QEMU common headers. It never broke
build because these two macros aren't use by the current code, but this
is about to change with the future addition of functional tests for 9P.

Also, these should have enclosing parenthesis to be usable in any
syntactical situation.

As suggested by Eric Blake, let's use UINT16_MAX and UINT32_MAX to address
both issues.
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p.h

@@ -99,8 +99,8 @@ enum p9_proto_version {
     V9FS_PROTO_2000L = 0x02,
 };
 
-#define P9_NOTAG    (u16)(~0)
-#define P9_NOFID    (u32)(~0)
+#define P9_NOTAG    UINT16_MAX
+#define P9_NOFID    UINT32_MAX
 #define P9_MAXWELEM 16
 
 #define FID_REFERENCED          0x1