can: slcan: export slcan_ethtool_ops and remove slcan_set_ethtool_ops()

The function slcan_set_ethtool_ops() does one thing: populate
net_device::ethtool_ops. Instead, it is possible to directly assign
this field and remove one function call and slightly reduce the object
size. To do so, export slcan_ethtool_ops so it becomes visible to
sclan-core.c.

This patch reduces the footprint by 14 bytes:

| $ ./scripts/bloat-o-meter drivers/net/can/slcan/slcan.{old,new}.o
| drivers/net/can/slcan/slcan.o
| add/remove: 0/1 grow/shrink: 1/0 up/down: 15/-29 (-14)
| Function                                     old     new   delta
| slcan_open                                  1010    1025     +15
| slcan_set_ethtool_ops                         29       -     -29
| Total: Before=11115, After=11101, chg -0.13%

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20220727104939.279022-2-mailhol.vincent@wanadoo.fr


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>