Input: i8042 - tease apart interrupt handler

In preparation to using guard notation when acquiring mutexes and
spinlocks factor out handling of active multiplexing mode from
i8042_interrupt().

Link: https://lore.kernel.org/r/20240905041732.2034348-13-dmitry.torokhov@gmail.com


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