irqchip/sl28cpld: Replace irqchip mask_invert with unmask_base

Remove use of the deprecated mask_invert flag. Inverted mask
registers (where a '1' bit enables an IRQ) can be described more
directly as an unmask register.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221112152701.41990-1-aidanmacdonald.0x0@gmail.com