Add pci_to_bdf() helper function.

This function is useful when setting the OPTIONROM_BDF_1/2 defs.
diff --git a/src/pci.h b/src/pci.h
index ab7cb9e..81ac629 100644
--- a/src/pci.h
+++ b/src/pci.h
@@ -15,6 +15,9 @@
 static inline u8 pci_bdf_to_fn(u16 bdf) {
     return bdf & 0x07;
 }
+static inline u16 pci_to_bdf(int bus, int dev, int fn) {
+    return (bus<<8) | (dev<<3) | fn;
+}
 
 void pci_config_writel(u16 bdf, u32 addr, u32 val);
 void pci_config_writew(u16 bdf, u32 addr, u16 val);