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

When polled without the use of IRQ, FT5x06 registers may return
undefined initial data, causing unwanted touches or event spamming.
A simple way to filter this out is to suppress touches until the
TD_STATUS register changes for the first time.

Increase the delay before first polling to 300ms, to avoid
transient I2C read flakiness that seems to occur after reset.

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