media: msi2500: assign SPI bus number dynamically

SPI bus number must be assigned dynamically for each device, otherwise it
will crash when multiple devices are plugged to system.

Reported-and-tested-by: syzbot+c60ddb60b685777d9d59@syzkaller.appspotmail.com

Cc: stable@vger.kernel.org
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/usb/msi2500/msi2500.c

@@ -1230,7 +1230,7 @@ static int msi2500_probe(struct usb_interface *intf,
 	}
 
 	dev->master = master;
-	master->bus_num = 0;
+	master->bus_num = -1;
 	master->num_chipselect = 1;
 	master->transfer_one_message = msi2500_transfer_one_message;
 	spi_master_set_devdata(master, dev);