drivers/pcie: do not verify configured IRQ

Some firmwares (looking at you, slimbootloader) don't set the registers
in PCI configuration space to indicate the IRQ routing, so we remove
the check that verifies that the user and firmware agree on IRQ number.

Also eliminate the return value of pcie_irq_enable() since no one uses
it and we can't return a meaningful value any longer.

Signed-off-by: Charles Youse <charles.youse@intel.com>