net: mdio: ipq8064: enlarge sleep after read/write operation

With the use of the qca8k dsa driver, some problem arised related to
port status detection. With a load on a specific port (for example a
simple speed test), the driver starts to behave in a strange way and
garbage data is produced. To address this, enlarge the sleep delay and
address a bug for the reg offset 31 that require additional delay for
this specific reg.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>