net: bridge: fix br_stp_enable_bridge comment

br_stp_enable_bridge() does take the br->lock spinlock. Fix its wrongly
pasted comment and use the same as br_stp_disable_bridge().

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>