net: aquantia: when cleaning hw cache it should be toggled

>From HW specification to correctly reset HW caches (this is a required
workaround when stopping the device), register bit should actually
be toggled.

It was previosly always just set. Due to the way driver stops HW this
never actually caused any issues, but it still may, so cleaning this up.

Fixes: 7a1bb494

 ("net: aquantia: fix potential IOMMU fault after driver unbind")
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>