spi: sun4i: Poll for rxfifo to be filled up

To drain rx fifo the fifo need to poll for how much data has
been filled up in rx fifo.

To achieve this, the current code is using wait_for_bit logic
on control register with exchange burst mode mask, which is not
a proper way of waiting for fifo filled up.

So, add code for polling rxfifo to be filled up using fifo
status register.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>