delta_tx and delta_rx in the SFP information are differences from
the values in the "calibration" SFP model. Thus they are signed
not uint32_t.  Fortunately nobody used negative values so far.

I also renamed the fields, to ensure the compiler would find all uses.

(there are other irrelevant changes in upstream headers; this commits
puts them in sync again).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>