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