r6040: fix link checking with switches

The current link checking logic only works for one port, which is not correct
for swiches were multiple ports can have different link status. As a result
we would only check for link status on port 1 of the switch. Move the calls
to mii_check_media in r6040_timer which will be polling a single PHY chip
correctly and assume link is up for switches.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>