[syncd] Fix the error log while running lua plugin (sonic-net#3974)

What I did

Fix sonic-net#24342

Why I did it

Variable is uninitialized which is causing the error logs mentioned in the issue during startup

Author: vivekrnv

orchagent/port_rates.lua

@@ -145,7 +145,8 @@ local function compute_rate(port)
     logit(initialized)
 
     -- FEC BER
-    local fec_corr_bits, fec_uncorr_frames, maxT
+    local fec_corr_bits, fec_uncorr_frames
+    local maxT = -1
     local fec_corr_bits_ber_new, fec_uncorr_bits_ber_new = -1, -1
     -- HLD review suggest to use the statistical average when calculate the post fec ber
     local rs_average_frame_ber = 1e-8