ft5x06_ts: Don't use BTN_TOUCH when using multi-touch mode

Oddly, the XINPUT driver appears to need this bit advertised
in order to recognize the device as a touch screen, but
doesn't need BTN_TOUCH events.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
ft5x06_ts: simplify through request_threaded_irq

The implementation of the interrupt handler as a delayed work
queue was overly complicated.

A threaded irq makes much more sense.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
ft5x06_ts: remove unused buttons field from struct ft5x06_ts

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
ft5x06_ts: ignore junk at POR

At power-on-reset, the touch status registers are frequently
garbage. Ignore them if the touch controller isn't currently
signalling readiness through the interrupt line.

See 5f7cc2677 from Android branch.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>