net: ftgmac100: fix potential NULL pointer access in ftgmac100_phy_disconnect

After the call to phy_disconnect() netdev->phydev is reset to NULL.
So fixed_phy_unregister() would be called with a NULL pointer as argument.
Therefore cache the phy_device before this call.

Fixes: e24a6c87 ("net: ftgmac100: Get link speed and duplex for NC-SI")
Cc: stable@vger.kernel.org
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Link: https://patch.msgid.link/2b80a77a-06db-4dd7-85dc-3a8e0de55a1d@gmail.com


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