V4L/DVB (12805): tm6000: Fix a memory leak at tm6000-video

if a transfer buffer allocation fails, the last allocated urb is leaked
(it hasn't been stored in dev->urb[] yet so tm6000_uninit_isoc misses
it). The patch also includes a small typo fix.

Thanks to Florin Malita <fmalita@gmail.com> for pointing this.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>