Various bug fixes.

[lopsided98]

This PR contains fixes for various bugs I ran into while using this crate, separated into different commits. Here is a summary:

• Channel needs to be Send to be used as an rtic resource. I think this is safe, but I'm not totally sure what the potential safety issues are in this context.
• Interrupt flags need to be cleared by setting the relevant bits, not clearing them.
• TransferDescriptor was missing functions to adjust the beat size.
• TransferDescriptor's implementation of Default called itself, leading to infinite recursion
• DMA trigger source definitions were wrong
  • SAMD5x trigger sources were missing Sercom2Tx and Sercom3Rx, causing all subsequent source IDs to be wrong.
  • Trigger sources were not defined for the SAMD21 (the SAMD51's were incorrectly used instead)

[proman21]

Thanks for submitting this PR! I've already made some of the changes you have in my local copy, but I never pushed them until now. Specifically, commits bd4d9a3 and c02e592, which add the (get|set)_beat_size methods and fix the Default recursion problem. I'd recommend you cherry-pick the other commits into a new branch, and submit another PR with that branch. I'm happy with the rest of the changes, so once you do that I'll approve it ASAP.