correct detection of USB header type
this fixes a regression for linux USB headers without 64 bytes extension

Change-Id: Icb79d983dac6dca25d04657d3324286c73d31f29
Reviewed-on: https://code.wireshark.org/review/3906
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>