net: fec_mxc: Fix corruption of device tree blob

Modifying content of dev->name leads to the device tree corruption
because it points to the node name located there.

Signed-off-by: Daniel Strnad <strnadda@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>