Fix compile warnings about unused variables

Fix compile warnings about unused variables

sysfs.c: In function 'sysfs_read_vpd':
sysfs.c:569:43: warning: unused parameter 'd' [-Wunused-parameter]
 static int sysfs_read_vpd(struct pci_dev *d, int pos, byte *buf, int len)
                                           ^
sysfs.c:569:50: warning: unused parameter 'pos' [-Wunused-parameter]
 static int sysfs_read_vpd(struct pci_dev *d, int pos, byte *buf, int len)
                                                  ^~~
sysfs.c:569:61: warning: unused parameter 'buf' [-Wunused-parameter]
 static int sysfs_read_vpd(struct pci_dev *d, int pos, byte *buf, int len)
                                                             ^~~
sysfs.c:569:70: warning: unused parameter 'len' [-Wunused-parameter]
 static int sysfs_read_vpd(struct pci_dev *d, int pos, byte *buf, int len)
                                                                      ^~~