lan743x: fix for potential NULL pointer dereference with bare card

lan743x: fix for potential NULL pointer dereference with bare card

This is the 3rd revision of the patch fix for potential null pointer dereference
with lan743x card.

The simpliest way to reproduce: boot with bare lan743x and issue "ethtool ethN"
commant where ethN is the interface with lan743x card. Example:

$ sudo ethtool eth7
dmesg:
[  103.510336] BUG: kernel NULL pointer dereference, address: 0000000000000340
...
[  103.510836] RIP: 0010:phy_ethtool_get_wol+0x5/0x30 [libphy]
...
[  103.511629] Call Trace:
[  103.511666]  lan743x_ethtool_get_wol+0x21/0x40 [lan743x]
[  103.511724]  dev_ethtool+0x1507/0x29d0
[  103.511769]  ? avc_has_extended_perms+0x17f/0x440
[  103.511820]  ? tomoyo_init_request_info+0x84/0x90
[  103.511870]  ? tomoyo_path_number_perm+0x68/0x1e0
[  103.511919]  ? tty_insert_flip_string_fixed_flag+0x82/0xe0
[  103.511973]  ? inet_ioctl+0x187/0x1d0
[  103.512016]  dev_ioctl+0xb5/0x560
[  103.512055]  sock_do_ioctl+0xa0/0x140
[  103.512098]  sock_ioctl+0x2cb/0x3c0
[  103.512139]  __x64_sys_ioctl+0x84/0xc0
[  103.512183]  do_syscall_64+0x33/0x80
[  103.512224]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  103.512274] RIP: 0033:0x7f54a9cba427
...

Previous versions can be found at:
v1:
initial version
    https://lkml.org/lkml/2020/10/28/921

v2:
do not return from lan743x_ethtool_set_wol if netdev->phydev == NULL, just skip
the call of phy_ethtool_set_wol() instead.
    https://lkml.org/lkml/2020/10/31/380

v3:
in function lan743x_ethtool_set_wol:
use ternary operator instead of if-else sentence (review by Markus Elfring)
return -ENETDOWN insted of -EIO (review by Andrew Lunn)

Signed-off-by: Sergej Bauer <sbauer@blackbox.su>
Link: https://lore.kernel.org/r/20201101223556.16116-1-sbauer@blackbox.su
Signed-off-by: Jakub Kicinski <kuba@kernel.org>