power: regulator: tps65941: Cannot test unsigned for being negative

power: regulator: tps65941: Cannot test unsigned for being negative

In tps65941_buck_val and tps65941_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>