spi: take the SPI IO-mutex in the spi_set_cs_timing method

this patch takes the io_mutex to prevent an unprotected HW
register modification in the set_cs_timing callback.

Fixes: 4cea6b8c ("spi: add power control when set_cs_timing")
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Link: https://lore.kernel.org/r/20210508060214.1485-1-leilk.liu@mediatek.com


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