spi: xtensa-xtfpga: fix register endianness

XTFPGA SPI controller has native endian registers.
Fix register acessors so that they work in big-endian configurations.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org