net: aquantia: introduce fwreq mutex

Some of FW operations could be invoked simultaneously,
from f.e. ethtool context and from service service activity work.
Here we introduce a fw mutex to secure and serialize access
to FW logic.

Signed-off-by: Nikita Danilov <ndanilov@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>