staging: wilc1000: use list_head to maintain 'rxq_entry_t elements in rx queue

Make use of 'list_head' data structure to maintain the rx buffer queue.
Modified wilc_wlan_rxq_add() to add the element at the tail by using
list_head API and wilc_wlan_rxq_remove() to remove the element from
head.

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>