media: ddbridge: fix possible buffer overflow in ddb_ports_init()

Report from smatch:

  drivers/media/pci/ddbridge/ddbridge-core.c:2659 ddb_ports_init() error: buffer overflow 'dev->port' 32 <= u32max

Fix by making sure "p" is greater than zero before checking for
"dev->port[].type == DDB_CI_EXTERNAL_XO2".

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>