net: dsa: mv88e6xxx: introduce wait mask routine

The current mv88e6xxx_wait routine is used to wait for a given mask
to be cleared to zero. However in some cases, the driver may have
to wait for a given mask to be of a certain non-zero value.

Thus provide a generic wait mask routine that will be used to implement
the current mv88e6xxx_wait function, and use it to wait for 88E6185
PPU states.

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