dev, misc: Fixing "may be used unitialized" compilation error

When compiling using "scons build/X86/base", "error: 'tx_queue_size'
may be used uninitialized in this function" is received (cc1plus:
all warnings treated as errors). tx_queue_size is now initialized
to zero to avoid this compilation error.

Change-Id: I0e2a4fd9ad6053c4c4124c83da9a7919778bcc52
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21399
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/dev/net/etherlink.cc b/src/dev/net/etherlink.cc
index 448bb88..e250639 100644
--- a/src/dev/net/etherlink.cc
+++ b/src/dev/net/etherlink.cc
@@ -242,7 +242,7 @@
         parent->schedule(doneEvent, event_time);
     }
 
-    size_t tx_queue_size;
+    size_t tx_queue_size = 0;
     if (optParamIn(cp, base + ".tx_queue_size", tx_queue_size)) {
         for (size_t idx = 0; idx < tx_queue_size; ++idx) {
             Tick tick;