Wait at end of ehal 0.2 blocking SPI writes

[ianrrees]

Summary

Discovered while tracking down an issue with the PyGamer display identified here - basically for non-duplex SPIs, the embedded-hal 0.2 blocking writes were returning as soon as the last word was written to the SERCOM, not waiting until it had been shifted out. This resulted in the command/data line for the LCD controller being toggled out of sync with the SPI data stream.

I've tested this only with the PyGamer example.

Checklist

• CHANGELOG.md for the BSP or HAL updated
• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

[sajattack]

clippy's a little bit unhappy with this, but once you resolve that it should be good to go.