[libata] fix build error on drivers/ata/pata_legacy.c

fix those errors:

drivers/ata/pata_legacy.c: In function ‘pdc_data_xfer_vlb’:
drivers/ata/pata_legacy.c:289: error: ‘ap’ undeclared (first use in this function)
drivers/ata/pata_legacy.c:289: error: (Each undeclared identifier is reported only once
drivers/ata/pata_legacy.c:289: error: for each function it appears in.)
drivers/ata/pata_legacy.c: At top level:
drivers/ata/pata_legacy.c:869: error: ‘ATA_PFLAG_PIO32_CHANGE’ undeclared here (not in a
+function)
make[2]: *** [drivers/ata/pata_legacy.o] Error 1
make[1]: *** [drivers/ata] Error 2

Signed-off-by: Zhenwen Xu <helight.xu@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>