ata: declare ata_port_info structures as const

ata_port_info structures are either copied to other objects or their
references are stored in objects of type const. So, ata_port_info
structures having similar usage pattern can be made const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>