regulator: ad5398: Fix min/max current limit boundary checking

It is ok to request current limit with min_uA < chip->min_uA and
max_uA > chip->max_uA.

We need to set min_uA = chip->min_uA if (min_uA < chip->min_uA),
this ensures the equation to calcuate selator does not return negative number.

Also set max_uA = chip->max_uA if (max_uA > chip->max_uA), as suggested by
Sonic.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>