openpower-pels: clean up various compile warnings

Compile warnings observed when compiling parts of the
openpower-pels (or corresponding tests) under stricter
compiler warning flags of Meson.

Issues fixed:
    - many unused parameters
    - invalid case fall-through
    - excess semi-colons
    - incorrect 'const' on return-by-value type
    - removal of variable length array in test case
    - uncaught return from 'system' call in test case

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8af69184042cf8661d1307a02ecf3afcab4724a1
diff --git a/extensions/openpower-pels/pldm_interface.cpp b/extensions/openpower-pels/pldm_interface.cpp
index 8fa7c18..c891a4a 100644
--- a/extensions/openpower-pels/pldm_interface.cpp
+++ b/extensions/openpower-pels/pldm_interface.cpp
@@ -147,7 +147,7 @@
     }
 }
 
-int iidCallback(sd_bus_message* msg, void* data, sd_bus_error* err)
+int iidCallback(sd_bus_message* msg, void* data, sd_bus_error* /*err*/)
 {
     auto* interface = static_cast<PLDMInterface*>(data);
     interface->instanceIDCallback(msg);
@@ -254,7 +254,7 @@
     }
 }
 
-void PLDMInterface::receive(IO& io, int fd, uint32_t revents)
+void PLDMInterface::receive(IO& /*io*/, int fd, uint32_t revents)
 {
     if (!(revents & EPOLLIN))
     {