hwmon: qpnp-current: fix converting offset and gain to voltage

The current driver forces a value of 0uV for offset and always
lands up with 17.857mV for gain. The raw values for offset and
gain are correct however.

Update the driver to do a proper conversion of raw offset and
gain to values in microvolts.

Change-Id: I19eda7a4f0cc8ab6af0d31e01e1168d99f00bf7a
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>