fix(PeriphDrivers): Fix SPI RevA1 DMA Bug When Transaction Length is Less Than the FIFO Size

[Jake-Carter]

Description

Fixes #879 by adding some extra logic to handle the edge case where the transaction length is less than the FIFO size. In this case, there is nothing left for the DMA to do after the FIFO is pre-loaded. The previous implementation would enable the DMA with a transfer count of 0, so the callbacks would never get triggered.

AFAIK the FIFO pre-load works around some hardware issues on some parts and is required.

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