net: bcmgenet: Don't claim WOL when its not available

Some of the bcmgenet platforms don't correctly support WOL, yet
ethtool returns:

"Supports Wake-on: gsf"

which is false.

Ideally if there isn't a wol_irq, or there is something else that
keeps the device from being able to wakeup it should display:

"Supports Wake-on: d"

This patch checks whether the device can wakup, before using the
hard-coded supported flags. This corrects the ethtool reporting, as
well as the WOL configuration because ethtool verifies that the mode
is supported before attempting it.

Fixes: c51de7f3

 ("net: bcmgenet: add Wake-on-LAN support code")
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220310045535.224450-1-jeremy.linton@arm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>