libata and starting/stopping ATAPI floppy devices

Prevent libata from starting/stopping non-ATA devices (like ATAPI floppy
drives) as they don't seem to like it:

sd 1:0:1:0: [sdb] Starting disk
ata2.01: configured for PIO2
sd 1:0:1:0: [sdb] Result: hostbyte=0x00 driverbyte=0x08
sd 1:0:1:0: [sdb] Sense Key : 0x2 [current]
sd 1:0:1:0: [sdb] ASC=0x3a ASCQ=0x0

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>