Fix unreliable writes to cyw43.

We use a pio and dma to write to the cyw43 chip using spi. Normally you
write an address and then read the data from that address, so the pio
program does does a write then read.

If you just want to write data in the case of uploading firmware we
use the fdebug_tx_stall flag to work out if the pio has stalled waiting
to write more data.

The theory is that this flag will also get set if the bus is busy. So
we mistakenly think a write to cyw43 has completed.

Wait for the dma write to complete before waiting for the pio to stall.

Fixes raspberrypi#2206

Author: peterharperuk

src/rp2_common/pico_cyw43_driver/cyw43_bus_pio_spi.c

@@ -308,11 +308,13 @@ int cyw43_spi_transfer(cyw43_int_t *self, const uint8_t *tx, size_t tx_length, u
 
         dma_channel_configure(bus_data->dma_out, &out_config, &bus_data->pio->txf[bus_data->pio_sm], tx, tx_length / 4, true);
 
+        pio_sm_set_enabled(bus_data->pio, bus_data->pio_sm, true);
+        dma_channel_wait_for_finish_blocking(bus_data->dma_out);
+
         uint32_t fdebug_tx_stall = 1u << (PIO_FDEBUG_TXSTALL_LSB + bus_data->pio_sm);
         bus_data->pio->fdebug = fdebug_tx_stall;
-        pio_sm_set_enabled(bus_data->pio, bus_data->pio_sm, true);
         while (!(bus_data->pio->fdebug & fdebug_tx_stall)) {
-            tight_loop_contents(); // todo timeout
+            tight_loop_contents();
         }
         __compiler_memory_barrier();
         pio_sm_set_enabled(bus_data->pio, bus_data->pio_sm, false);