904d0e78 ("PCI: designware: Add irq_create_mapping()") resulted in
pre-allocated irq descs.  Problem was that in assign_irq() these descs were
explicitly allocated and hence also freed, resulting in a crash.  We also
need to clear the entire irq range in teardown.  With this commit the
teardown basically does exactly the opposite of what was done in setup.

The crash this fixes looks like:

  Unable to handle kernel NULL pointer dereference at virtual address 00000020
  PC is at dw_msi_teardown_irq+0x40/0x118
  LR is at trace_hardirqs_on_caller+0xf4/0x1c0
  Backtrace:
  [<802c401c>] (dw_msi_teardown_irq+0x0/0x118) from [<802c1844>] (arch_teardown_msi_irq+0x3c/0x40)
  [<802c1808>] (arch_teardown_msi_irq+0x0/0x40) from [<802c1a08>] (default_teardown_msi_irqs+0x68/0x84)
  [<802c19a0>] (default_teardown_msi_irqs+0x0/0x84) from [<802c1a34>] (arch_teardown_msi_irqs+0x10/0x14)
  [<802c1a24>] (arch_teardown_msi_irqs+0x0/0x14) from [<802c1ad0>] (free_msi_irqs+0x98/0x144)
  [<802c1a38>] (free_msi_irqs+0x0/0x144) from [<802c2570>] (pci_disable_msi+0x48/0x60)
  [<802c2528>] (pci_disable_msi+0x0/0x60) from [<7f0057d4>] (sxdma_irq_free+0x44/0x48 [sxdma])

[bhelgaas: add crash info]
Tested-by: Mohit Kumar <mohit.kumar@st.com>
Signed-off-by: Bjørn Erik Nilsen <ben@datarespons.no>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Jingoo Han <jg1.han@samsung.com>