media: rtl28xxu: add missing sleep before probing slave demod

Slave demod needs some time to wake up otherwise it may not respond to the
following probe commands. This problem manifested randomly on my Astrometa
DVB-T2 dongle.
Signed-off-by: Mario Hros <git@reversity.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/usb/dvb-usb-v2/rtl28xxu.c

@@ -552,6 +552,9 @@ static int rtl2832u_read_config(struct dvb_usb_device *d)
 		if (ret)
 			goto err;
 
+		/* slave demod needs some time to wake up */
+		msleep(20);
+
 		/* check slave answers */
 		ret = rtl28xxu_ctrl_msg(d, &req_mn88472);
 		if (ret == 0 && buf[0] == 0x02) {