net: dsa: mv88e6xxx: introduce wait bit routine

Many portions of the driver need to wait until a given bit is set
or cleared. Some busses even have a specific implementation for this
operation. In preparation for such variant, implement a generic Wait
Bit routine that can be used by the driver core functions.

This allows us to get rid of the custom implementations we may find
in the driver. Note that for the EEPROM bits, BUSY and RUNNING bits
are independent, thus it is more efficient to wait independently for
each bit instead of waiting for their mask.

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