V4L/DVB (4342): Fix ext_controls align on 64 bit architectures

u64 is aligned as 128bits on x86_64 architetures, requiring an special
handling to ioctls that depends on v4l2_ext_control. 
Let's fix this before ext controls go to kernel mainstream to avoid one 
more compat32 stuff.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>