[media] saa7146: Create a device name before it's used

request_irq() uses it, tries to create a procfs file with an empty name
otherwise.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=83771



Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>