genirq/irqdesc: Decrease indentation level in __irq_get_desc_lock()

There is a conditional that covers all the code for the entire function.
Invert it and decrease indentation level. This also helps for further
changes to be clearer and tidier.

[ tglx: Removed line breaks ]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250416114122.2191820-2-andriy.shevchenko@linux.intel.com