input: prevent the caller from sleeping in sysworkq

Sleeping in sysworkq is a very bad idea, it can trash any hope of
realtimeness at best, deadlock the whole system at worse.

Add a check to input_report to downgrade the event to K_NO_WAIT
automatically when called from the sysworkq, similarly to what's done by
other APIs (netbuf and bluetooth), though only log if messages are
actually dropped.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>