spi: sprd: Fix the possible negative value of BIT()

When enabling the ADI hardware channels, if the channel id is 31,
then we will get one negative value -1 for BIT() macro, which will
write incorrect value to register.

Fixes: 7e2903cb

 ("spi: Add ADI driver for Spreadtrum platform")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>