power: regulator: max77686: Don't use switch() on bools

With gcc-5.3 we get a warning for using switch() on a bool type.
Rewrite these sections as if/else and update the one section that was
using 1/0 instead of true/false.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>