net: ftmac100: add an opportunity to get ethaddr from the platform

This driver always generated a random ethernet address. Leave it as a
fallback solution, but add a call to platform_get_ethdev_address().
Handle EPROBE_DEFER returned from platform_get_ethdev_address() to
retry when EEPROM is ready.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220824151724.2698107-1-saproj@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>