spi/spi-atmel: BUG: fix doesn' support 16 bits transfers using PIO

Fix using PIO transfer mode only support 8 bits transfer, doesn't support 16 bits.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
[wenyou.yang@atmel.com: submit the patch]
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>