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>