media: dvb_ca_en50221: Used a helper variable

Used a helper variable "struct dvb_ca_slot *sl" instead of
"ca->slot_info[slot]". This reduces the line length and simplifies
code reading.

Fixed also "-strict" checks in this patch:
- Comparison to NULL written as "!<var>".
Signed-off-by: Jasmin Jessich <jasmin@anw.at>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>