bridge: Correctly clamp MAX forward_delay when enabling STP

Commit be4f154d
	bridge: Clamp forward_delay when enabling STP
had a typo when attempting to clamp maximum forward delay.

It is possible to set bridge_forward_delay to be higher then
permitted maximum when STP is off.  When turning STP on, the
higher then allowed delay has to be clamed down to max value.

CC: Herbert Xu <herbert@gondor.apana.org.au>
CC: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Reviewed-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>