libata-sff: kill unused prototype and make ata_dev_select() static

ata_irq_on() was renamed to ata_sff_irq_on() and exported a while ago
but prototype for the original function lingered in
drivers/ata/libata.h.  Kill it.  Also, ata_dev_select() is only used
inside drivers/ata/libata-sff.c.  Make it static.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>