hwmon: qpnp-adc-current: Fix current resolution units

Existing units returned for IADC current is in mA when
using external rsense. Fix it to return the units in uA
to keep it consistent with current read from internal rsense.

Change-Id: Ia9792ea4d894500829011db5510363b4e6e14616
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>