ofproto: Fix wildcard masking with nw_tos.

Later patches rely on the DSCP and ECN masks to be properly set when
the fields are read.  Also, avoid reading nw_tos if tunnel's inner
packet is not IP.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>