libertas: reduce explicit references to priv->cur_cmd->cmdbuf

We have a local variable 'resp' which we use for this. So use it,
instead of typing the whole thing.

In preparation for actually using priv->upld_buf for the responses
instead...

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>