[media] bt8xx: remove needless check

As reported by smatch:
	drivers/media/pci/bt8xx/dst_ca.c:323 ca_get_message() warn: this array is probably non-NULL. 'p_ca_message->msg'
	drivers/media/pci/bt8xx/dst_ca.c:498 ca_send_message() warn: this array is probably non-NULL. 'p_ca_message->msg'

Those two checks are needless/useless, as the ca_msg struct is
declared as:
typedef struct ca_msg {
        unsigned int index;
        unsigned int type;
        unsigned int length;
        unsigned char msg[256];
} ca_msg_t;

So, if the p_ca_message pointer is not null, msg will also be
not null.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>