[libata] pata_artop: kill gcc warning

gcc complains thusly:

drivers/ata/pata_artop.c: In function ‘artop_init_one’:
drivers/ata/pata_artop.c:429: warning: ‘info’ may be used uninitialized in this function

While this warning is indeed bogus, even with improved static analysis
and value range propagation, gcc will probably never be able to detect
this.

Add a BUG_ON() to trap invalid driver_data entries in the PCI table.

Signed-off-by: Jeff Garzik <jeff@garzik.org>