media: saa7134: fix incorrect use to determine if list is empty

'dev' will *always* be set by list_for_each_entry().
It is incorrect to assume that the iterator value will be NULL if the
list is empty.

Instead of checking the pointer it should be checked if
the list is empty.

Fixes: 79dd0c69 ("V4L: 925: saa7134 alsa is now a standalone module")
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>