mei: Add include of <linux/irqreturn.h>

We currently include <linux/irqreturn.h> in <linux/pci.h>, but I'm about to
remove that from linux/pci.h, so add explicit includes where needed.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>