power: supply: core: Use library interpolation

The power supply core appears to contain two open coded
linear interpolations. Use the kernel fixpoint arithmetic
interpolation library function instead.

Cc: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>