V4L/DVB (5326): Allow to set tuner_config in attach inform

This patch move the assignment of the tuner config and the callback
before the check whether it is called in the attach inform.
This solves a module load order issue

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>