Input: gscps2 - return IRQ_NONE when interrupt is not handled

gscps2_interrupt() is registered with IRQF_SHARED. Unconditionally
returning IRQ_HANDLED when no data was pending on any GSC PS/2 port
masks unhandled interrupts on the shared interrupt line and prevents the
kernel core spurious interrupt detector from identifying runaway
interrupt storms.

Have gscps2_read_data() return whether any bytes were read, accumulate
the handled status in gscps2_interrupt(), and return IRQ_RETVAL(handled).

Reported-by: sashiko-bot@kernel.org
Assisted-by: LLM
Link: https://patch.msgid.link/20260830-gscps2-v1-5-c733d4cae7f9@gmail.com
Acked-by: Helge Deller <deller@gmx.de>
Tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>