ide-acpi: remove taskfile_load_raw()

* taskfile_load_raw() is used only by do_drive_set_taskfiles()
  so inline it there.

While at it:
- rename 'args' variable to 'task'
- remove struct taskfile_array
- do ide_acpigtf check early
- use REGS_PER_GTF

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>