hwmon: qpnp-adc-current: Fix external rsense value

When external rsense is selected the api used to
calculate the rsense needs to return back. Instead
it recalculates the rsense using the trim procedure
used for calcuating the internal rsense. Fix it
by returning the value that is used for the external
rsense.

Change-Id: Icde61ca2c95d1b4ab64d9f6db2fc97ac4e47daab
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>