When the ctl_core_mode[1:0] field of the
CFG_CORE_MODE_REG is set to 2'b00, the ILKNF subsystem supports up to 300 Gb/s of
Interlaken. In this case, only half of the RS-FEC instances are needed for the
Interlaken function, and the remaining RS-FEC instances are available for use for
FEC-only encode+decode functions. Specifically, the last three FEC instances (FEC3,
FEC4, FEC5) are available for FEC-only.