net: fec_mxc: Remove unneeded eth_device arg from fec_get_hwaddr

fec_get_hwaddr never used eth_device argument, hence removed.

Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>