staging: wilc1000: use list_head to maintain 'txq_entry_t' elements of tx queue

Use list_head data structure for the doubly linked list instead of own
implementation.
Only 'txq_head' is required, so removed the txq_tail pointer from
'wilc' structure.

Following functions are modified to provide data using list_head API's
wilc_wlan_txq_remove()
wilc_wlan_txq_remove_from_head()
wilc_wlan_txq_add_to_tail()
wilc_wlan_txq_add_to_head()
wilc_wlan_txq_get_first()
wilc_wlan_txq_get_next()

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>