spi: atmel: use endian agnostic IO

Use the endian agnositc IO functions instead of the __raw ones for when
the driver is in use on big-endian systems.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>