mcs7830: Implement link state detection

Add .status callback that detects link state changes.
Tested with MCS7832CV-AA chip (9710:7830, identified as rev.C by the driver).
Fixes https://bugzilla.kernel.org/show_bug.cgi?id=28532

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: David S. Miller <davem@davemloft.net>