The disable bits for the ethernet interfaces between FPGA and HPS are read
and configured, but never written back.
The configuration itself doesn't make that much sense however. So instead of
writing it back to the register, remove the whole read-modify operation altogether.

Reported-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>