input: touchscreen: edt-ft5x06: Suppress bogus data on startup

When polled without the use of IRQ, FT5x06 registers may hold
undefined data. Filter out the initial static values.

Increase the delay before first polling to 400ms, to avoid
transient I2C read flakiness that seems to occur after reset.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>