net: ftmac100: add mii read and write callbacks

Register mii_bus with read and write callbacks to allow the 'mii'
command to work. Use a timeout of 10 ms to wait for the R/W
operations to complete.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Rick Chen <rick@andestech.com>