power: fg-alg: return -1 instead of -EINVAL on error

Align our code to the return value found in other versions of the patch
"power: qpnp-qg: Add TTF_VALID param for valid TTF/TTE reporting".

This stops the time_to_full_now attribute from returning -22 (-EINVAL)
when the battery is fully charged which VTS is not happy about.

Issue: FP3-A12#4
Test: run vts -m VtsHalHealthV2_1TargetTest -t PerInstance/HealthHidlTest#getHealthInfo_2_1/0_default
Change-Id: Ib36d74a4cbed0c125217bd15c07de5eff2c42659