Input: i8042 - set up shared ps2_cmd_mutex for AUX ports

The commit 40974618 ("Input: i8042 - break load dependency ...")
correctly set up ps2_cmd_mutex pointer for the KBD port but forgot to do
the same for AUX port(s), which results in communication on KBD and AUX
ports to clash with each other.

Fixes: 40974618 ("Input: i8042 - break load dependency ...")
Reported-by: Bruno Wolff III <bruno@wolff.to>
Tested-by: Bruno Wolff III <bruno@wolff.to>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>