regulator: gpio-regulator: Fix finding of smallest value

Commit 4dbd8f63f07a (regulator: gpio-regulator: Set the smallest
voltage/current in the specified range) forgot to set the newly
introduced best_val.

Therefore it stayed always at INT_MAX thus breaking the setting
of the voltage.

Included is also an init value for target, as warnings about
a possibly uninitialised target started appearing with this fix.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>