spi: sprd: Simplify the transfer function

We can move the hardware spinlock protection into the ADI read/write
functions to simplify the sprd_adi_transfer_one() function. Moreover
this optimization can also help to access PMIC without considering
the hardware spinlock using sprd_adi_read/write() functions.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>