How to find out capacity for network interfaces?
You're looking in the wrong place.
Look at /sys/class/net/<device>/speed.
You can check the interface capacity in following ways.
dmesg | grep eth0mii-tool -v eth0ethtool eth0
Note: Change the device name according to yours.
In general, you could find this out using a tool such as strace:
sudo strace -e trace=file -f -s128 lshw -class network