media: vcam: set capture buffer sequence numbers

[grawis]

This patch sets the V4L2 capture buffer sequence number for each completed
frame.

Before this change, applications using VIDIOC_DQBUF always observed
buf.sequence == 0, even though frames were being streamed successfully. This
made it impossible for userspace to track frame order through the standard
V4L2 buffer metadata.

The fix adds a per-device sequence counter, resets it when streaming starts,
and assigns it to each completed capture buffer before calling
vb2_buffer_done().

Tested with a local MMAP streaming test:

Frame  Index  Sequence  Bytesused  Timestamp(us)
    0      0         0     921600     ...
    1      1         1     921600     ...
    2      2         2     921600     ...
    3      0         3     921600     ...
Sequence: PASS
Timestamp: PASS
Bytesused: PASS

Fixes #54.

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Summary by cubic

Set V4L2 capture buffer sequence numbers so VIDIOC_DQBUF returns increasing buf.sequence and apps can track frame order. Fixes the issue where all frames reported sequence 0.

• Bug Fixes
  • Added per-device sequence counter to vcam_device, reset on stream start.
  • Set vb.sequence before vb2_buffer_done() in both submit paths.

^{Written for commit 6e18f1f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}