power: regulator: palmas: Cannot test unsigned for being negative

power: regulator: palmas: Cannot test unsigned for being negative

In palmas_smps_val and palmas_ldo_val hex is an unsigned variable being
assigned the return value from a function that returns int. Change hex
to be an int so that the following test for an error as a negative value
will work as expected.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>