net: bcmgenet: Clear RGMII_LINK upon link down

Clear the RGMII_LINK bit upon detecting link down to be consistent with
setting the bit upon link up. We also move the clearing of the
out-of-band disable to the runtime initialization rather than for each
link up/down transition.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221118213754.1383364-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>