we can have multiple UARTs on the same thread sharing the same DMA TX
channel. That can lead to deadlock with blocking locks on DMA. This
makes UART requests for DMA locks non-blocking to fix the issue
this fixes a problem with the ICM-20789 on I2C where a reboot while the bus is active
leaves the IMU in a dead state where it can't be recovered without a power cycle.
Andrew Tridgell