hiddev_table[] is an array of pointers.  the minor number is used as an
offset.  hiddev minors start either with zero, or with 96.  If they start
with 96, the offset must be reduced by HIDDEV_MINOR_BASE because only 16
minors are available.  unplugging a hiddevice will zero data outside the
hiddev_table array.

this was spotted by Takashi Iwai.