datapath: Use HAVE_PROTO_DATA_VALID when defining vswitch_skb_checksum_setup

The purpose of the non-empty variant of vswitch_skb_checksum_setup is to
synchronise the proto_data_valid and proto_csum_blank fields into the
standard skb csum/ip_summed fields, therefore it is more correct to key
off of HAVE_PROTO_DATA_VALID.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>