libata-acpi: implement _GTF command filtering

libata-acpi: implement _GTF command filtering

Implement _GTF command filtering which can be controlled by
libata.acpi_filter kernel parameter.  Currently SETXFER and LOCK
commands are filtered.

libata configures transfer mode by itself and _GTF SETXFER commands
can potentially disrupt device configuration.  _GTM/_STM mechanism
can't handle hotplugging too well and when _GTF is executed,
controller is in PIO0 rather than the mode _STM configured.

Note that detecting SET MAX LOCK requires looking at the previous
command.  This adds a bit to code complexity.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>