xhci: switch to pci_alloc_irq_vectors

Use the modern API to request MSI or MSI-X interrupts, which allows us to
get rid of the msix_entries array, as well as cleaning up the cleanup
code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>