boot: zephyr: Fix misaligned #else for `irq_lock`

So in PR #1089 I misaligned the `#else` used for the `irq_lock` as it
should be aligned with `CONFIG_MCUBOOT_CLEANUP_ARM_CORE`. So that
`irq_lock`is called when we don't call `__disable_irq` from arm core
clean up code.

Signed-off-by: Sigvart Hovland <sigvart.hovland@nordicsemi.no>