usbnet: mcs7830: rework link state detection

Even with the quirks in commit dabdaf0c

 (mcs7830: Fix link state
detection) there are still spurious link-down events for some chips
where the false link-down events count go over a few hundreds.

This patch takes a more conservative approach and only looks at
link-down events where the link-down state is not combined with other
states (e.g. half/full speed, pending frames in SRAM or TX status
information valid). In all other cases we assume the link is up.

Tested on MCS7830CV-DA (USB ID 9710:7830).

Cc: Ondrej Zary <linux@rainbow-software.org>
Cc: Michael Leun <lkml20120218@newton.leun.net>
Cc: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>