wilc1000: let wilc_mac_xmit() return NETDEV_TX_OK

The method ndo_start_xmit() is defined as returning an 'netdev_tx_t',
which is a typedef for an enum type defining 'NETDEV_TX_OK' but this
driver returns '0' instead of 'NETDEV_TX_OK'.

Fix this by returning 'NETDEV_TX_OK' instead of '0'.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200629104009.84077-1-luc.vanoostenryck@gmail.com