Add a 'continuous' option for usb charging which enables
the "linear" charging mode of the twl4030.

Linear charging does a good job with not-so-reliable power sources.
Auto mode does not work well as it switches off when voltage drops
momentarily.  Care must be taken not to over-charge.

It was used with a bike hub dynamo since a year or so. In that case
there are automatically charging stops when the cyclist needs a break.
Original-by: NAndreas Kemnade <andreas@kemnade.info>
Signed-off-by: NNeilBrown <neil@brown.name>
Signed-off-by: NSebastian Reichel <sre@kernel.org>