irqchip/gic-v5: iwb: Fix iounmap probe failure path

The 0-day bot reported that on the failure path the driver iounmap()s IWB
resources that are managed through devm_ioremap(), which is clearly wrong
because the driver would end up unmapping the MMIO resource twice on
probing failure.

Fix this by removing the error path altogether and by letting devres manage
the iounmapping on clean-up.

Fixes: 695949d8 ("irqchip/gic-v5: Add GICv5 IWB support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20250801-gic-v5-fixes-6-17-v1-1-4fcedaccf9e6@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202508010038.N3r4ZmII-lkp@intel.com