net: dsa: mv88e6xxx: wait for AVB Busy bit

The AVB is not an indirect table using an Update bit, but a unit using
a Busy bit. This means that we must ensure that this bit is cleared
before setting it and wait until it gets cleared again after writing
an operation. Reflect that.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>