net: dsa/mv88e6352: make mv88e6352_wait generic

Some busy bits are available in the global register 1, such as the ATU
Busy bit. We may want to use this function to wait for them to change,
so add a new parameter to mv88e6352_wait() instead of hard-coding
REG_GLOBAL2.

In the meantime, since the REG_READ() macro already checks for error,
remove the redundant check for ret < 0.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>