irqchip/irq-brcmstb-l2: Set IRQ_LEVEL for level triggered interrupts

When support for the level triggered interrupt controller flavor was
added with c0ca7262, we forgot to update the flags to be set to
contain IRQ_LEVEL. While the flow handler is correct, the output from
/proc/interrupts does not show such interrupts as being level triggered
when they are, correct that.

Fixes: c0ca7262

 ("irqchip/brcmstb-l2: Add support for the BCM7271 L2 controller")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221216230934.2478345-2-f.fainelli@gmail.com