[pfcwd] Convert polling interval from ms to us in LUA scripts (#1908)

**What I did**
Converted polling interval from milliseconds to microseconds in PFCWD LUA scripts.

**Why I did it**
PFCWD storm detection and restoration LUA scripts require values in microseconds.
Due to recent changes polling interval is now passed in milliseconds by "FlexCounter".
* sonic-net/sonic-sairedis#878

So need to convert values to microseconds (as it was before) in order to make PFCWD working,

**How I verified it**
Ran PFCWD tests from sonic-mgmt.

orchagent/pfc_detect_mellanox.lua

@@ -1,12 +1,12 @@
 -- KEYS - queue IDs
 -- ARGV[1] - counters db index
 -- ARGV[2] - counters table name
--- ARGV[3] - poll time interval
+-- ARGV[3] - poll time interval (milliseconds)
 -- return queue Ids that satisfy criteria
 
 local counters_db = ARGV[1]
 local counters_table_name = ARGV[2]
-local poll_time = tonumber(ARGV[3])
+local poll_time = tonumber(ARGV[3]) * 1000
 
 local rets = {}
 

orchagent/pfc_restore.lua

@@ -1,12 +1,12 @@
 -- KEYS - queue IDs
 -- ARGV[1] - counters db index
 -- ARGV[2] - counters table name
--- ARGV[3] - poll time interval
+-- ARGV[3] - poll time interval (milliseconds)
 -- return queue Ids that satisfy criteria
 
 local counters_db = ARGV[1]
 local counters_table_name = ARGV[2]
-local poll_time = tonumber(ARGV[3])
+local poll_time = tonumber(ARGV[3]) * 1000
 
 local rets = {}