HV:debug:fix "expression is not Boolean"

MISRA C explicit required expression should be boolean when
in branch statements (if,while...).

Signed-off-by: Huihuang Shi <huihuang.shi@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/debug/printf.c

@@ -28,7 +28,7 @@ static int charout(int cmd, const char *s, int sz, void *hnd)
 	/* fill mode */
 	else {
 		*nchars += sz;
-		while (sz--)
+		while ((sz--) != 0)
 			console_putc(*s);
 	}