bq27541: Add current_now property

The current is returned by the charger as a twos
complement signed integer in milliamps.  Applications
such as CurrentWidget expect this to be microamps.

Signed-off-by: Andrew Dodd <atd7@cornell.edu>
Change-Id: I053b3d1f24ec04391e982fdcb4b3486241c3aa34
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>