[media] av7110_hw: Don't use dynamic static allocation

Dynamic static allocation is evil, as Kernel stack is too low, and
compilation complains about it on some archs:
	drivers/media/pci/ttpci/av7110_hw.c:510:1: warning: 'av7110_fw_cmd' uses dynamic stack allocation [enabled by default]
Instead, let's enforce a limit for the buffer.
In the specific case of this driver, the maximum fw command size
is 6 + 2, as checked using:
	$ git grep -A1 av7110_fw_cmd drivers/media/pci/ttpci/
So, use 8 for the buffer size.
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/pci/ttpci/av7110_hw.c

@@ -40,6 +40,14 @@
 
 #define _NOHANDSHAKE
 
+/*
+ * Max transfer size done by av7110_fw_cmd()
+ *
+ * The maximum size passed to this function is 6 bytes. The buffer also
+ * uses two additional ones for type and size. So, 8 bytes is enough.
+ */
+#define MAX_XFER_SIZE  8
+
 /****************************************************************************
  * DEBI functions
  ****************************************************************************/
@@ -488,11 +496,18 @@ static int av7110_send_fw_cmd(struct av7110 *av7110, u16* buf, int length)
 int av7110_fw_cmd(struct av7110 *av7110, int type, int com, int num, ...)
 {
 	va_list args;
-	u16 buf[num + 2];
+	u16 buf[MAX_XFER_SIZE];
 	int i, ret;
 
 //	dprintk(4, "%p\n", av7110);
 
+	if (2 + num > sizeof(buf)) {
+		printk(KERN_WARNING
+		       "%s: %s len=%d is too big!\n",
+		       KBUILD_MODNAME, __func__, num);
+		return -EINVAL;
+	}
+
 	buf[0] = ((type << 8) | com);
 	buf[1] = num;