smsc: replace WARN_ON() with WARN_ON_SMP()

spin_is_locked() always return false in uniprocessor configuration and therefore it
would be advise to repalce with WARN_ON_SMP().

Signed-off-by: Sanjeev Sharma <Sanjeev_Sharma@mentor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>