[media] lmedm04: don't write to buffer without a mutex

usb_buffer not inside mutex lock, waiting caller can alter buffer.
 Static added to lme2510_exit and lme2510_exit_int.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>