This reverts commit 1b2503fc.

Unfortunately this fix regresses console handling on MIPS Malta;
since the mux ctrl-a b bug is not a regression since 2.11, we
take the conservative approach and just drop it from 2.12.

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>