power: qpnp-charger: don't call ADC API in ISR

the qpnp_adc_measure API can not be called in ISR
as it invokes usleep.

Fix this by scheduling a worker to request the
ADC.

Change-Id: I90b8d6220a1a97d7faa07e48ae1783d116f4c6ee
CRs-Fixed: 470068
Signed-off-by: David Keitel <dkeitel@codeaurora.org>