cx82310_eth: wait for firmware to become ready

When the device is powered up, some (older) firmware versions fail to work
properly if we send commands before the boot is complete (everything is OK
when the device is hot-plugged). The firmware indicates its ready status by
putting the link up.
Newer firmwares delay the first command so they don't suffer from this problem.
They also report the link being always up.

Wait for firmware to become ready (link up) before sending any commands and/or
data.

This also allows lowering CMD_TIMEOUT value to a reasonable time.

Tested with 4.1.0.9 (old) and 4.1.0.30 (new) firmware versions.

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