net: ti: am65-cpsw: Fix buffer overflow

The device name is a concatenation of the device node name of the cpsw
device and of the device node name of the port. In my case that is

  ethernet@8000000
  port@1

First the buffer is really too small, but more importantly, there is no
boundary check. Use snprintf() and increase the buffer size.

Fixes: 38922b1f ("net: ti: am65-cpsw: Add support for multi port independent MAC mode")
Signed-off-by: Michael Walle <mwalle@kernel.org>