Remap Acer WMI touchpad toggle key to F21 used by X

Remap Acer WMI touchpad toggle key to F21 used by X

Currently, Acer WMI driver generates KEY_F22 but this
will soon change to KEY_TOUCHPAD_TOOGLE.

X has defined F21 for the purpose of touchpad toggle and other
udev keymaps align with this meaning.  Patch aligns Acer WMI
hotkey drivers with F21.

Tested on Acer TravelMate 8572 notebook using acer-wmi driver.

Cc: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>