Input: psmouse - fix data race in __ps2_command

The data race happens on ps2dev->cmdcnt and ps2dev->cmdbuf contents.
__ps2_command reads that data concurrently with the interrupt handler. As
the result, for example, if a response arrives just after the timeout,
__ps2_command can copy out garbage from ps2dev->cmdbuf but then see that
ps2dev->cmdcnt is 0 and return success.

Stop the interrupt handler with serio_pause_rx() before reading the
results.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>