The check code is a one byte code that can be used to verify
that the first 64 (0-63) bytes of serial information in the
SFP transceiver is valid.Returns yes if checksum matches, no
if checksum doesn't match. For XFPs, The check code shall be
the low order 8 bits of the sum of the contents of all the
bytes from byte 128 to byte 190, inclusive.