hw/gpio/aspeed: Fix clear incorrect interrupt status for GPIO index mode

hw/gpio/aspeed: Fix clear incorrect interrupt status for GPIO index mode

The interrupt status field is W1C, where a set bit on read indicates an
interrupt is pending. If the bit extracted from data is set it should
clear the corresponding bit in reg_value. However, if the extracted
bit is clear then the value of the corresponding bit in reg_value
should be unchanged.

SHARED_FIELD_EX32() extracts the interrupt status bit from the write
(data). reg_value is set to the set's interrupt status, which means
that for any pin with an interrupt pending, the corresponding bit is
set. The deposit32() call updates the bit at pin_idx in the
reg_value, using the value extracted from the write (data).

The result is that if multiple interrupt status bits
were pending and the write was acknowledging specific one bit,
then the all interrupt status bits will be cleared.
However, it is index mode and should only clear the corresponding bit.

For example, say we have an interrupt pending for GPIOA0, where the
following statements are true:

   set->int_status == 0b01
   s->pending == 1

Before it is acknowledged, an interrupt becomes pending for GPIOA1:

   set->int_status == 0b11
   s->pending == 2

A write is issued to acknowledge the interrupt for GPIOA0. This causes
the following sequence:

   reg_value == 0b11
   pending == 2
   s->pending == 0
   set->int_status == 0b00

It should only clear bit 0 in index mode and the correct result
should be as following.

   set->int_status == 0b11
   s->pending == 2

   pending == 1
   s->pending == 1
   set->int_status == 0b10

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Suggested-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>