gpio: dln2: Fix interrupts when replugging the device

When replugging the device the following message shows up:

gpio gpiochip2: (dln2): detected irqchip that is shared with multiple gpiochips: please fix the driver.

This also has the effect that interrupts won't work.
The same problem would also show up if multiple devices where plugged in.

Fix this by allocating the irq_chip data structure per instance like other
drivers do.

I don't know when this problem appeared, but it is present in 5.10.

Cc: <stable@vger.kernel.org> # 5.10+
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>