power: pm8921-charger: PM8917 USB trimming fix

PM8917 has slightly different trimming setup than
other PMIC platforms and requires a 7th bit.

Fix this by reading another scratch register to obtain
this value.

CRs-Fixed: 428106
Change-Id: I71482f9a4430a8bbcd02c954a415ff3fab2a0c8f
Signed-off-by: David Keitel <dkeitel@codeaurora.org>