twl4030_charger: decrease USB charging current to 550mA

At 560mA, it will cause some USB host controllers to trigger their over-current
protection failsafes, usually resulting in a total loss of power to the USB
line. Since in general, USB 2.0 ports can't be assumed to have more than 500mA
of power available on a given port, further reductions may be needed, but this
should do for now.