hwmon: pm8921-adc: request's done late into suspend

-Add checks in the driver for suspend_noirq/resume_noirq
to prevent clients from invoking the driver for ADC requests
after suspend has begun. This will lead to interrupts being
lost since the request is done very late into suspend.
-Obtain a wakelock when an ADC request is occuring.

Change-Id: I19c9b2f22b92d2729baa8b31f14966410e39af14
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>