Use constants instead of vwr->STATS_LEN in some cases.

Use constants instead of vwr->STATS_LEN in some cases.

In some cases, we know, based on the FPGA code, what vwr->STATS_LEN is,
so use that #define.

While we're at it, replace some hardcoded numbers that represent the
statistics trailer length with the appropriate #define.

Also, combine two identical case arms for Ethernet into one.

Change-Id: I0bdea8e5aab146094ad21fa7e67dca2913da688b
Reviewed-on: https://code.wireshark.org/review/672
Reviewed-by: Guy Harris <guy@alum.mit.edu>