[media] tm6000: fix an uninitialized variable

tm6000_poll could use an uninitialized buf pointer. Move the buf-handling
code inside the 'if' that initializes the buf pointer.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>