libata: init ata_print_id to 0

When comparing the dmesg between 3.4-rc3 and 3.4-rc4 I found the
following differences:

 -ata1: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff100 irq 47
 -ata2: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff180 irq 47
 -ata3: DUMMY
 +ata2: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff100 irq 47
 +ata3: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff180 irq 47
  ata4: DUMMY
  ata5: DUMMY
 -ata6: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff380 irq 47
 +ata6: DUMMY
 +ata7: SATA max UDMA/133 abar m2048@0xf9fff000 port 0xf9fff380 irq 47

The change of numbering comes from commit 85d6725b ("libata:
make ata_print_id atomic") that changed lines like

	ap->print_id = ata_print_id++;
		to
	ap->print_id = atomic_inc_return(&ata_print_id);

As the latter behaves like ++ata_print_id, we must initialize
it to zero to start the numbering from one.

Signed-off-by: Tero Roponen <tero.roponen@gmail.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>