r8169: prevent bit sign expansion error in mdio_write

Oops.

The current code does not like being given an u16 with the highest
bit set as an argument to mdio_write. Let's enforce a correct range of
values for both the register address and value (resp. 5 and 16 bits).

The callers are currently left as-is.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>