staging: rtl8712: Fix return type for implementation of ndo_start_xmit

staging: rtl8712: Fix return type for implementation of ndo_start_xmit

CFI (Control Flow Integrity) is a safety feature allowing the system to
detect and react should a potential control flow hijacking occurs. In
particular, the Forward-Edge CFI protects indirect function calls by
ensuring the prototype of function that is actually called matches the
definition of the function hook.

Since Linux now supports CFI, it will be a good idea to fix mismatched
return type for implementation of hooks. Otherwise this would get
cought out by CFI and cause a panic.

Use enums from netdev_tx_t as return value instead, then change return
type to netdev_tx_t.

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Link: https://lore.kernel.org/r/20220905130230.11230-1-guozihua@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>