nrf5x: Fix DMA access race condition
nrf5x: Fix DMA access race condition

In multi-thread mode starting DMA in thread mode was
prone to race condition resulting in infinite loop.
It may happen on single core CPU with strict priority based
tasks scheduler where ready high prio task never yields to
ready low prio task (Mynewt).

Sequence that failed (T1 - low priority task, T2 - high priority task)
- T1 called start_dma()
- T1 set _dcd.dma_running (DMA not started yet, context switch happens)
- T2 took CPU and saw that _dcd.dma_running is set, so waits for _dcd.dma_running to be 0
- T1 never gets CPU again, DMA is not started T2 waits forever

OSAL mutex resolves problem of DMA starting from thread-context.

hathach/tinyusb,hathach/tinyusb,hathach/tinyusb