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

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>