sound/pipewire: remove duplicate increment

[MatiasVara]

Summary of the PR

For capturing, pipewire backend is incrementing the position in the buffer, however, this is already done by write_input() thus resulting in incrementing two times the position. This commit removes the incrementation that happens in pipewire.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR have Signed-Off-By trailers (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[stefano-garzarella]

CI failure is unrelated. I'll open another PR to update the vhost-device-vsock dependency

[MatiasVara]

After this change in some places we use buf_pos, in others buffer.pos, I would try to unify it (maybe removing buf_pos at all)

I just figured out. I removed everywhere.