V4L/DVB (8047): bt8xx: i2c structure templates clean-up

Clean up the use of structure templates in bttv-i2c. For one thing, a
real template is supposed to be read-only. And in some cases it's more
efficient to initialize the few fields we need individually.

This clean-up shrinks bttv-i2c.o by 29% (x86_64).

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>