sccb-ng.c: correct address byte-swapping in Write16 routines

[dhalbert]

Description

• Fixes incorrect double reg byte-swapping in SCCB_Write16() and SCCB_Write_Addr16_Val16().
• Minor other fixes in sccb-ng.c:
  • Remove unused macros.
  • Remove spurious return.

Related

• Fixes new I2C driver sccb-ng.c is mistakenly byte-swapping reg in SCCB_Write16() and SCCB_Write_Addr16_Val16() #689.
  @playduck FYI

Testing

Tested with an OV5640 camera, which uses 16-bit addresses

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.