interface and endpoint descriptors can't possibly appear in a
get descriptor response on their own, only as part of a configuration
descriptor

Change-Id: I2d70ab1d1adc19cbc0fe8c2c835071153a40ae7d
Reviewed-on: https://code.wireshark.org/review/3730
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>