Improve detection of extended QSFP diagnostics.

The standards in the QSFP diagnostics area are not clear when the
additional measurements are present or not. Use a valid temperature
reading as an indicator for the presence of voltage and TX/RX power
measurements.

MFC after: 1 week
Sponsored by: Mellanox Technologies
Tested by: Netflix
Differential Revision: https://reviews.freebsd.org/D5391
Reviewed by: gallatin