libertas: Zero 'pdata_size' field in cmd_ctrl_node reliably.

Otherwise, lbs_process_rx_command() will take the new path for
lbs_cmd() responses, when it shouldn't.

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