staging: wilc1000: use 'void' return for wilc_wlan_txq_add_to_head()

Use 'void' return for wilc_wlan_txq_add_to_head() as its always
return '0' value.

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