Fill FMT buffer size field in order to inform app which will be
used streaming buffer size. Currently driver doesn't allow buffer
size value proposed by application.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>