drm/rockchip: fix for mailbox read validation.

This is basically the same fix as in
commit fa68d4f8 ("drm/rockchip: fix for mailbox read size")
but for cdn_dp_mailbox_validate_receive function.

See patchwork.kernel.org/patch/10671981/ for details.
Signed-off-by: NDamian Kos <dkos@cadence.com>
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1542640463-18332-1-git-send-email-dkos@cadence.com

drivers/gpu/drm/rockchip/cdn-dp-reg.c

@@ -113,7 +113,7 @@ static int cdp_dp_mailbox_write(struct cdn_dp_device *dp, u8 val)
 
 static int cdn_dp_mailbox_validate_receive(struct cdn_dp_device *dp,
 					   u8 module_id, u8 opcode,
-					   u8 req_size)
+					   u16 req_size)
 {
 	u32 mbox_size, i;
 	u8 header[4];