octeontx2-af: RPM: fix stale FCFEC counters

The corrected words register(FCFECX_VL0_CCW_LO)/Uncorrected words
register (FCFECX_VL0_NCCW_LO) of FCFEC counter has different LMAC
offset which needs to be accessed differently.

Fixes: 84ad3642 ("octeontx2-af: Add FEC stats for RPM/RPM_USX block")
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>