TPM: do not set device description if probe fails

device_set_desc should be called only if driver probes successfully.

Approved by: mw(mentor)
Reviewed by: mw, kd
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35998

(cherry picked from commit 31ee3a33aa612294db302828d4d1f6f0c83bef4c)