[libata] allow ATAPI to be enabled with new atapi_enabled module option

[libata] allow ATAPI to be enabled with new atapi_enabled module option

ATAPI is getting close to being ready.  To increase exposure, we enable
the code in the upstream kernel, but default it to off (present
behavior).  Users must pass atapi_enabled=1 as a module option (if
module) or on the kernel command line (if built in) to turn on
discovery of their ATAPI devices.