Change ABI of mii_readreg() and mii_writereg()

[Masanobu SAITOH <msaitoh%execsw.org@localhost>]

 Hi.

 Our mii(4)'s mii_readreg() and mii_writereg() are defined as below:

> /*
>  * Callbacks from MII layer into network interface device driver.
>  */
> typedef int (*mii_readreg_t)(device_t, int, int);
> typedef void (*mii_writereg_t)(device_t, int, int, int);

 There is no formal way to tell any error (e.g. protocol error or timeout).
By this flaw, we had some bugs and fixed them many times.

 On some driver, if the return value of mii_readreg() is all 0 or all 1,
it regards as error. It's not good because some MII PHY registers are
counter.

 In 802.3 spec also says that the PHY shall not respond to read/write
transaction to the unimplemented register(22.2.4.3), so the device timeout
is exist in real. It can be used to detect whether a register is implemented
or not (if the register conforms to the spec).

 Some Ethernet controller or system are required to take a semaphore
while accessing the PHY. It might be failed to get the semaphore.

So, I propose to change the API as follows
(I also change read/write register value from int to uint16_t because
all of the register is 16bit (and it should be unsigned)):

int (*mii_readreg_t)(device_t, int, int, uint16_t *);
int (*mii_writereg_t)(device_t, int, int, uint16_t);

 OK?

--
-----------------------------------------------
                SAITOH Masanobu (msaitoh%execsw.org@localhost
                                 msaitoh%netbsd.org@localhost)