net: korina: fix kfree of rx/tx descriptor array

kmalloc returns KSEG0 addresses so convert back from KSEG1
in kfree. Also make sure array is freed when the driver is
unloaded from the kernel.

Fixes: ef11291b

 ("Add support the Korina (IDT RC32434) Ethernet MAC")
Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>