irqchip/irq-vt8500: Split up ack/mask functions

vt8500_irq_mask() really does the ACK for edge triggered interrupts and the
MASK for level triggered interrupts.  Edge triggered interrupts never
really are masked as a result, and there is unnecessary reading of the
status register before the ACK even though it's write-one-to-clear.

Split it up into a proper standalone vt8500_irq_ack() and an unconditional
vt8500_irq_mask().

No Fixes tag added, as it has survived this way for 15 years and nobody
complained, so apparently nothing really used edge triggered interrupts
anyway.

[ tglx: Tabularize the irqchip struct initializer ]

Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250506-vt8500-intc-updates-v2-1-a3a0606cf92d@gmail.com