Fix unused variable... [-Werror,-Wunused-const-variable]

Use DATA_FRAG_FLAGS, disabled unused NACK_FLAGS

Change-Id: I2d3dda2c3ab1ebbd66e32bf5ec1b62b521a7140d
Reviewed-on: https://code.wireshark.org/review/3968
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>