Cyw43 PIO SPI always busy waits, even for large transfers

[esky-software]

The function cyw43_spi_transfer() busy waits on the transfer DMA channel(s), even for large transfers.
This is far from optimal in an RTOS or other asynchronous environment.

We might use DMA interrupts to handle the completion of the transfer, but this would require some restructuring of the driver code.

More simply, we can estimate the duration of the transfer, and if it's larger than a threshold call cyw43_delay_us() to sleep via the async context before starting the busy wait for the end of the transfer.