net/rxrpc/ar-key.c: drop negativity check on unsigned value

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80611
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>