dpif-netdev: Modify code comments for function parse_tcp_flags. #2593

Job	Run time
osx clang --disable-ssl	1m 52s
linux gcc --disable-ssl	2m 1s
linux clang --disable-ssl	2m 13s
linux gcc c99	2m 8s
linux clang c99	2m 7s
linux gcc test 4.14	8m 1s
linux clang test 4.14 asan	12m 30s
linux gcc test --enable-shared	9m 45s
linux clang test --enable-shared	9m 54s
linux gcc test dpdk	14m 24s
linux clang test dpdk	14m 7s
linux gcc test -ljemalloc	7m 35s
linux clang test -ljemalloc	7m 18s
linux gcc 5.8 5.5 5.4 4.19	5m 34s
linux clang 5.8 5.5 5.4 4.19	5m 57s
linux gcc 4.14 4.9 4.4	3m 59s
linux clang 4.14 4.9 4.4	4m 15s
linux gcc afxdp 5.4	2m 55s
linux clang afxdp 5.4	2m 56s
linux gcc dpdk --enable-shared	9m 45s
linux clang dpdk --enable-shared	9m 46s
linux gcc dpdk-shared	8m 30s
linux clang dpdk-shared	8m 57s
linux gcc dpdk-shared --enable-shared	9m 28s
linux clang dpdk-shared --enable-shared	9m 5s
linux gcc m32 --disable-ssl	2m 5s
linux gcc deb	6m 19s
	3h 3m 26s

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