ietp(4): Load dummy HID report descriptor if mangled

Some Elantech trackpads have a mangled HID report descriptor, which
reads as having an incorrect input size (i.e. < IETP_REPORT_LEN_LO).
If the input size is incorrect, load a dummy report descriptor.

Reviewed by: wulf
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38387