V4L/DVB (7394): saa7134: add number of devices check

This patch fixes reported problems when trying to add a 9th device into a
system.

Signed-off-by: Alan McIvor <alan.mcivor@reveal.co.nz>
Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>