PCI MSI: msi_desc->dev is always initialised

By passing the pci_dev into alloc_msi_entry() we can be sure that
the ->dev entry is always assigned and so we don't need to check it.
Also, we used kzalloc() so we don't need to initialise ->irq to 0.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>