twl4030_charger: allow fine control of charger current.

The twl4030 allows control of the incoming current.
Part of this control is a 'CGAIN' setting which doubles
the range for half the precision.  This control affects
several different current setting, so all need to be updated
at once when CGAIN is changed.

With this patch, all of these current setting are managed
by the driver, but most are left at their default settings.

The current drawn is set to 500mA if the allow_usb module parameter is
set, and to 100mA otherwise.
More fine control will appear in later patches.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Sebastian Reichel <sre@kernel.org>