spi: sprd: don't mark remove function as __exit

The __exit section is left out for built-in drivers, so a
'remove' callback must not be marked as such to avoid breaking when
we unbind a device at runtime. This was pointed out by kbuild:

`sprd_spi_remove' referenced in section `.data' of drivers/spi/spi-sprd.o: defined in discarded section `.exit.text' of drivers/spi/spi-sprd.o

Fixes: e7d973a3 ("spi: sprd: Add SPI driver for Spreadtrum SC9860")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>