regulator: gpio-regulator: Split setting of voltages and currents

Originally gpio-regulator used the first item of its state list
that matched the given voltage or current range.

Commit 4dbd8f63 (regulator: gpio-regulator: Set the smallest voltage/current
in the specified range) changed this, to make the selection independent of
the ordering of the state list.

But selecting the minimal value is only true for voltage regulators.
For current regulators the maximum in the given range should be
selected instead.

Therefore split the previous common selection function into specific
functions for voltage and current regulators.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>