net/designware: do explicit port selection for 1Gb mode

net/designware: do explicit port selection for 1Gb mode

Current implementation only sets "port select" bit for non-1Gb mode.
That works fine if GMAC has just exited reset state but we may as well
change connection mode in runtime. Then we'll need to reprogram GMAC for
that new mode of operation and if previous mode was 10 or 100 Mb and new
one is 1 Gb we'll need to reset port mode bit.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
cc: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>