twl4030_charger: reduce default charge_current for usb

I've recently encountered a PC that has USB ports that limit current
draw to ~600mA - if more is drawn it cuts VBUS out. Incidentally pandora
has charge_current set to ~600mA, so on that PC charging only lasted for
a few secs and went out. Reducing the current to way below 600mA makes
charging reliable from that PC USB ports, and ~7% slower charging should
not make that much difference.