[pci] Allow pci_vpd_init() return status to be ignored

Most xxx_init() functions are void functions with no failure cases.
Allow pci_vpd_init() to be used in the same way.  (Subsequent calls to
pci_vpd_read() etc. will fail if pci_vpd_init() fails.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/bus/pcivpd.c

 	unsigned int retries;
 	uint16_t flag;
 
+	/* Fail if no VPD present */
+	if ( ! cap )
+		return -ENOTTY;
+
 	/* Return cached value, if present */
 	if ( pci_vpd_cache_is_valid ( vpd ) &&
 	     ( vpd->cache.address == address ) ) {
 	unsigned int retries;
 	uint16_t flag;
 
+	/* Fail if no VPD present */
+	if ( ! cap )
+		return -ENOTTY;
+
 	/* Invalidate cache */
 	pci_vpd_invalidate_cache ( vpd );
 

src/include/ipxe/pcivpd.h

 	struct pci_vpd_cache cache;
 };
 
+/**
+ * Check for presence of PCI VPD
+ *
+ * @v vpd		PCI VPD
+ * @ret is_present	VPD is present
+ */
+static inline __attribute__ (( always_inline )) int
+pci_vpd_is_present ( struct pci_vpd *vpd ) {
+	return ( vpd->cap != 0 );
+}
+
 /**
  * Check if PCI VPD read cache is valid
  *