hwmon: pm8xxx-adc: Fix error handling conditions

a) Fix returning error code back to clients when pa_therm
power enable fails and call regulator_get once at init for
pa_therm instead of every enable call.
b) Fix checking for the pm8xxx_adc probe has succeeded before
allowing clients of MPP configure and read to issue requests.

CRs-Fixed: 338187
Change-Id: I4c14d8e63429779b611d550ca699a0822c01d1c9
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>