ofproto-dpif-xlate: Correct check for MPLS LSE

ofproto-dpif-xlate: Correct check for MPLS LSE

zero is a valid MPLS LSE so it is not valid check against
that value for MPLS LSE presence. Instead, check against
the flow's dl_type which should be an MPLS type if an LSE is present.

This problem appears to have been introduced by
b2dd70be133bf86c ("Native Set-Field action.").

Cc: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>