staging: wilc1000: fix to free allocated memory in wilc_add_ptk()

Free allocated memory in wilc_add_ptk() when it fails to enqueue the
command.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>