healthd: Fix negativity check after cast to unsigned enum

mapSysfsString return code is checked for negativity after being casted
to unsigned enum type, which will always be false. This is obviously
unintended behaviour.

Fixes tautological-unsigned-enum-zero-compare warning.

Bug: 72331526
Test: m
Change-Id: Icec76d7a1121cb56fd9d05feb70cede69954c322