Input: ucb1400 - move static function from header into core

it's a little too large for static line.
The ts is currently the only mainline user but Marek Vasut claims that
there is a battery driver in an ARM tree which also needs this function.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>