V4L/DVB: remove unneeded null check in anysee_probe()

Smatch complained because "d" is dereferenced first and then checked for
null later .  The only code path where "d" could be a invalid pointer is
if this is a cold device in dvb_usb_device_init().  I consulted Antti
Palosaari and he explained that anysee is always a warm device.

I have added a comment and removed the unneeded null check.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>