nrf: fix bug in SPI.Tx

There was what appears to be a race condition in the Tx function. While
it would work fine in many cases, when there were interrupts (such as
when using BLE), the function would just hang waiting for `EVENTS_READY`
to arrive.

I think what was happening was that the `spi.Bus.RXD.Get()` would start
the next transfer, which would complete (and generate an event) before
`EVENTS_READY` was reset to 0. The fix is easy: clear `EVENTS_READY`
before doing something that can trigger an event.

I believe I've seen this bug before on the PineTime but I couldn't find
the issue back then.

src/machine/machine_nrf.go

@@ -443,13 +443,13 @@ func (spi SPI) Tx(w, r []byte) error {
 			spi.Bus.TXD.Set(uint32(b))
 			for spi.Bus.EVENTS_READY.Get() == 0 {
 			}
-			_ = spi.Bus.RXD.Get()
 			spi.Bus.EVENTS_READY.Set(0)
+			_ = spi.Bus.RXD.Get()
 		}
 		for spi.Bus.EVENTS_READY.Get() == 0 {
 		}
-		_ = spi.Bus.RXD.Get()
 		spi.Bus.EVENTS_READY.Set(0)
+		_ = spi.Bus.RXD.Get()
 
 	default:
 		// write/read