media: pci: tw686x: no need to check 'next'

If 'done' is not NULL, then next can never be NULL, so just drop
the 'next' check.

This fixes this smatch warning:

drivers/media/pci/tw686x/tw686x-audio.c:62 tw686x_audio_irq() warn: can 'next' even be NULL?

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>