power: pmic: Use trailing_strtol() instead of a local function

Use the common function to obtain the number from the end of the string,
instead of a local function. Also tweak the position of a debug() statement.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>