net: aquantia: Make RX default frame size 2K

This correlates with default internet MTU. This also allows page
flip/reuse to be activated, since each allocated RX page now serves for
two frags/packets.

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