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

This is a refinement which replaces or should be squashed with
commit dd802fc0 from PR #5645
and the new commit message should be the following:

When polled without the use of IRQ, FT5x06 registers may return
undefined initial data, causing unwanted touches or event spamming.
To avoid this, whenever the initial number of touches is non-zero,
discard all touches until something changes.

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>