switchdev: change BUG_ON to WARN for attr set failure case

This particular BUG_ON condition was checking for attr set err in the
COMMIT phase, which isn't expected (it's a driver bug if PREPARE phase is
OK but COMMIT fails).  But BUG_ON() is too strong for this case, so change
to WARN().  BUG_ON() would be warranted if the system was corrupted beyond
repair, but this is not the case here.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>