wilc1000: Bring MAC address setting in line with typical Linux behavior

Linux network drivers normally disallow changing the MAC address when
the interface is up.  This driver has been different in that it allows
to change the MAC address *only* when it's up.  This patch brings
wilc1000 behavior more in line with other network drivers.  We could
have replaced wilc_set_mac_addr() with eth_mac_addr() but that would
break existing documentation on how to change the MAC address.
Likewise, return -EADDRNOTAVAIL (not -EINVAL) when the specified MAC
address is invalid or unavailable.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210303194846.1823596-1-davidm@egauge.net