v4l2-linux: Fix nv12 linesize

[kkartaltepe]

Description

unlike other YUV based formats nv12 chroma plane has the same padding as
the luma plane.
ref: https://01.org/linuxgraphics/gfx-docs/drm/media/uapi/v4l/pixfmt-nv12.html

Motivation and Context

Fixes nv12 capture
fixes #3506

How Has This Been Tested?

Tested using ffmpeg and v4l2loopback for capture.
ffmpeg -re -loop 1 -i ~/SMPTE_Color_Bars.png -vf 'format=nv12' -f v4l2 /dev/video2

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.