staging: wilc1000: handle freeing of key data in wilc_add_ptk()

Handle freeing of memory allocated to store the 'key' in wilc_add_ptk()
function. Once work completion notification is received, free the
memory allocated to avoid missing of free in work function sepecially
for error scenario.

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>