vppapigen: crc is a negative value for some messages when using python 2.7
vppapigen: crc is a negative value for some messages when using python 2.7

Type: fix

Signed-off-by: Mark Nelson <8920b7253b016659c4a48b970b86c43d5d795a45@cisco.com>
Change-Id: I4f121e49d3c05c21eed3fed2469bd88fc84e2271

FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp