lcd_cam: i8080 fix bad len calculation (missed from ReadBuffer update)

[liebman]

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Submission Checklist 📝

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Pull Request Details 📖

PR #1834 mised a change related to ReadBuffer in send_dma_async and as a result computes the len as twice the size it should be for 16bit.

Description

No need to multiply the len returned by ReadBuffer::read_buffer()

Testing

Local hub75 implementation.

[bjoernQ]

Thanks!

[JurajSadel]

Thanks!