MIIPHYBB: Return 0xFFFF if the PHY is not asserting TA.

This patch sets the returned value to 0xFFFF if the PHY does not exist
and does not assert Transfer Acknowledge. A NULL check for the value
pointer is also added for buffer overflow protection.

Without this patch 'mii info' will  show 'phantom' devices because the
value will be not be initialized and return with some random value.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>