media: cx18: Don't check for address of video_dev

Clang warns that the address of a pointer will always evaluated as true
in a boolean context.

drivers/media/pci/cx18/cx18-driver.c:1255:23: warning: address of
'cx->streams[i].video_dev' will always evaluate to 'true'
[-Wpointer-bool-conversion]
                if (&cx->streams[i].video_dev)
                ~~   ~~~~~~~~~~~~~~~^~~~~~~~~
1 warning generated.

Check whether v4l2_dev is null, not the address, so that the statement
doesn't fire all the time. This check has been present since 2009,
introduced by commit 21a278b8

 ("V4L/DVB (11619): cx18: Simplify the
work handler for outgoing mailbox commands")

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>