[media] stk1160: Handle urb allocation failure condition properly

When an urb buffer can't be allocated, the currently allocated
buffer count must be saved so they can properly released.
Moreover, it's sufficient to call stk1160_free_isoc to have
all urb buffers released.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>