net: aquantia: Change inefficient wait loop on fw data reads

B1 hardware changes behavior of mailbox interface, it has busy bit
always raised. Data ready condition should be detected by increment
of address register.

Old code has empty `for` loop, and that caused cpu overloads on B1
hardware. aq_nic_service_timer_cb consumed ~100ms because of that.

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