Enable SFF Manger on Arista linecards by default

SFF Manager in xcvrd needs to run to take non-cmis transceivers out of
lpmode as part of the fix for these transceivers remaining in down state.

device/arista/x86_64-arista_common/pmon_daemon_control_linecard.json

@@ -1,5 +1,6 @@
 {
     "skip_fancontrol": true,
-    "skip_pcied": true
+    "skip_pcied": true,
+    "enable_xcvrd_sff_mgr": true
 }