[efi] Provide space for storing the EFI driver name

Commit d7736fb ("[efi] Allow EFI to control PCI bus enumeration")
introduced a bug in which the EFI driver name became an
(uninitialised) pointer rather than an array.

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

src/include/ipxe/efi/efi_driver.h

 	/** Name */
 	const char *name;
 	/** EFI name */
-	CHAR16 *wname;
+	CHAR16 wname[32];
 	/** EFI driver binding protocol */
 	EFI_DRIVER_BINDING_PROTOCOL driver;
 	/** EFI component name protocol */