mp_map_gsi_to_irq() returns different codes if it fails.
intel_mid_pci_irq_enable() hides this under -EBUSY.

Return the actual failure code.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/1438161409-4671-3-git-send-email-andriy.shevchenko@linux.intel.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>