net: aquantia: Eliminate aq_nic structure abstraction

aq_nic_s was hidden in aq_nic_internal.h, that made it difficult to access
nic fields and structures from other modules.
This change moves aq_nic_s struct into aq_nic.h and thus makes it available
to other driver modules, mainly pci module and hw related module.

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