net: moxa: MAC address reading, generating, validity checking

This device does not remember its MAC address, so add a possibility
to get it from the platform. If it fails, generate a random address.
This will provide a MAC address early during boot without user space
being involved.

Also remove extra calls to is_valid_ether_addr().

Made after suggestions by Andrew Lunn:
1) Use eth_hw_addr_random() to assign a random MAC address during probe.
2) Remove is_valid_ether_addr() from moxart_mac_open()
3) Add a call to platform_get_ethdev_address() during probe
4) Remove is_valid_ether_addr() from moxart_set_mac_address(). The core does this

v1 -> v2:
Handle EPROBE_DEFER returned from platform_get_ethdev_address().
Move MAC reading code to the beginning of the probe function.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
CC: Yang Yingliang <yangyingliang@huawei.com>
CC: Pavel Skripkin <paskripkin@gmail.com>
CC: Guobin Huang <huangguobin4@huawei.com>
CC: Yang Wei <yang.wei9@zte.com.cn>
CC: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220818092317.529557-1-saproj@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>