fix typo in command value in drivers/net/phy/mdio-bitbang.

mdio-bitbang mentioned 10 for both read and write.
However mdio read opcode is 10 and write opcode is 01
Fixed comment.

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>