spi: armada100_spi: Remove unnecessary NULL test for dout and din

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Ajay Bhargav <ajay.bhargav@einfochips.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>