spi: dw: add check for Rx FIFO overflow

If even one byte is lost due to Rx FIFO overflow then we will never
exit the read loop. Because the (priv->rx != priv->rx_end) condition will
be always true.

Let's check if Rx FIFO overflow occurred and exit the read loop
in this case.

Signed-off-by: Maksim Kiselev <bigunclemax@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>