V4L/DVB (6684): Complement va_start() with va_end() + style fixes

Complement va_start() with va_end() + minor style fixes in the same function.

Signed-off-by: Richard Knutsson <ricknu-0@student.ltu.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>