[media] vivid: sdr cap: few enhancements

* Constify struct

* Fix comments

* Fix alignment

* Use modulus to transfer phase angles

* Correct float [-1.0, +1.0] to s8 [-128, 127] conversion

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>