fix(PeriphDrivers): Fix UART DMA Callbacks for UART1+

[Jake-Carter]

Description

Fixes #998

As the internal UART state struct was only initialized per used UART instance, attempting to use UART1 (or higher) with DMA auto handlers would never trigger the user's callback functions. The state struct's DMA channel values would be initialized to 0 instead of -1, which the drivers would assume was a valid channel acquisition.

This PR fixes the bug by initializing the UART state struct at run-time.

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