net: sockets_service: Always clear old events when registering

Clear all previous events when register is called a second time. This is
the same behavior as before, if the call happens with `len` equal or
greater then the previous call but in the case if `len` is less then the
first call, now, all events are cleared. This is more predictable behavior.

Signed-off-by: Cla Mattia Galliard <cla-mattia.galliard@zuehlke.com>