hwmon: qpnp-adc: Fix panic in ADC workqueue

During warm/soft reset it is seen during qpnp-adc-tm
driver interrupt registration there are pending
interrupts.

Register the ADC interrupt handler after initializing
the workqueue to handle potential cases where interrupt
is pending when an interrupt is requested and causes
panic because the workqueue is called when its not
initialized yet.

Change-Id: Iba88ab3c9f71dc94579ab85980f8cfd726f3c5c1
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>