staging: ks7010: fix ks_wlan_start_xmit()'s return type

staging: ks7010: fix ks_wlan_start_xmit()'s return type

The method ndo_start_xmit() is defined as returning an 'netdev_tx_t',
which is a typedef for an enum type, but the implementation in this
driver returns an 'int'.

Fix this by returning 'netdev_tx_t' in this driver too and
usind 'NETDEV_TX_OK' instead of 0 accordingly.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Link: https://lore.kernel.org/r/20200628183926.74908-1-luc.vanoostenryck@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>