usb: xhci: Check for XHCI_PLAT in xhci_cleanup_msix()

If CONFIG_PCI is enabled, make sure xhci_cleanup_msix()
doesn't try to free a bogus PCI IRQ or dereference an invalid
pci_dev when the xHCI device is actually a platform_device.

This patch should be backported to kernels as old as 3.9, that
contain the commit 52fb61250a7a132b0cfb9f4a1060a1f3c49e5a25
"xhci-plat: Don't enable legacy PCI interrupts."

Change-Id: I605644206bece0085bcc461932da13033e2bbb50
Signed-off-by: Jack Pham <jackp@codeaurora.org>