media: dvb_ca_en50221: avoid speculation from CA slot

As warned by smatch:
	drivers/media/dvb-core/dvb_ca_en50221.c:1392 dvb_ca_en50221_io_do_ioctl() warn: potential spectre issue 'ca->slot_info' [r] (local cap)

There's a potential of using a CAM ioctl for speculation.

The risk here is minimum, as only a small subset of DVB
boards have CI, with a CAM module installed. Also, exploiting
it would require a user capable of starting a DVB application.

There are probably a lot of easier ways to try to exploit.

Yet, it doesn't harm addressing it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>