can: rockchip_canfd: fix return type of rkcanfd_start_xmit()

can: rockchip_canfd: fix return type of rkcanfd_start_xmit()

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed. A
warning in clang aims to catch these at compile time, which reveals:

  drivers/net/can/rockchip/rockchip_canfd-core.c:770:20: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
    770 |         .ndo_start_xmit = rkcanfd_start_xmit,
        |                           ^~~~~~~~~~~~~~~~~~

->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
'netdev_tx_t', not 'int' (although the types are ABI compatible). Adjust
the return type of rkcanfd_start_xmit() to match the prototype's to
resolve the warning.

Fixes: ff60bfbaf67f ("can: rockchip_canfd: add driver for Rockchip CAN-FD controller")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20240906-rockchip-canfd-wifpts-v1-1-b1398da865b7@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>