This reverts commit 68da1664.

It turns out that the assertion about scope of regressions due to
always keeping keyboard controller in legacy mode was proven wrong.
There are laptops, such as Clevo W650SH, that only have internal
touchpad (no external PS/2 ports), that require active multiplexing
mode to switch the touchpad (Elantech) into native mode instead of
basic PS/2 emulation.
Reported-by: NRoel Aaij <roel.aaij@gmail.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>