twl4030_charger: allow changing end-of-charge currnet.

Allows the user to change the TWL4030's end-of-charge current parameter
via a sysfs file. If set to 400mA, it nicely solves the overcharging
problem on the pandora. Note that this commit also changes the format of
the charge_current sysfs file to be in micro-amperes, to make it
consistent with all the other sysfs parameters. Since it looks like this
file was only being used for debugging purposes, this shouldn't break
anybody's tools. (fingers crossed.)