regulator: mt6315: Fix function prototype for mt6315_map_mode

The .of_map_mode should has below function prototype:
	unsigned int (*of_map_mode)(unsigned int mode);

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20210530022109.425054-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>