cmdline: fix warning for unused return value

cmdline: fix warning for unused return value

When DPDK is compiled on Ubuntu with extra warnings turned on, there is a
warning about the return value from write() being unchecked. Rather than
having builds disable the warning, which may mask other cases we do care
about, we can add a dummy use of the return value in the code to silence it
in this instance.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>