spi: sprd: Fix the incorrect SPI register

The original code used an incorrect SPI register to initialize the SPI
controller in sprd_spi_init_hw(), thus fix it.

Fixes: e7d973a3

 ("spi: sprd: Add SPI driver for Spreadtrum SC9860")
Signed-off-by: Huanpeng Xin <huanpeng.xin@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/b4f7f89ec0fdc595335687bfbd9f962213bc4a1d.1575443510.git.baolin.wang7@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>