spi: dw: Fix Designware SPI loopback

The SPI_LOOP is set in spi->mode but not propagated to the register.
A previous patch removed the bit during a cleanup.

Fixes: e1bc2048 ("spi: dw: fix potential variable assignment error")
Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Link: https://lore.kernel.org/r/1572985330-5525-1-git-send-email-thor.thayer@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>