power: qpnp-charger: allow 0 mA iusbmax request

Currently 0 mA requests for qpnp_chg_iusbmax_set
are rejected.
Fix this by allowing 0 mA and configuring IUSB_MAX
to the lowest setting of 100 mA (0x00).

Change-Id: Iff1ee312e85e3fe33d6421630215196125c1136f
CRs-Fixed: 580614
Signed-off-by: David Keitel <dkeitel@codeaurora.org>