shell: Init structure and add the ARG_UNUSED macro

The shell_cmd structure has three parameters and only two were
initialized. So, this commit initializes the remaining parameter
and also adds the ARG_UNUSED macro to avoid compiler warnings.

Change-Id: I667a4e9839ec5acd526df0bb465beb2062518315
Signed-off-by: Flavio Santes <flavio.santes@intel.com>