hwmon: pm8xxx-adc: Remove wakelocks

The PM8xxx ADC interrupt is configured as a
wakeable interrupt. Therefore the system can suspend
and still have the end of conversion interrupt be
serviced.

Add a timeout in the wait_for_completion in case
the end of conversion event is not received. This
is to rule out any side affects if a missed interrupt
occurs.

Change-Id: Iad36ad4fb7e550e563f0bf7803d3e5e861b7aacd
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>