ide: add generic packet command representation ide_atapi_pc

This new struct unifies ide{-floppy,-tape,-scsi}'s view of a packet command. For now,
it represents the common denominator between the three drivers while adding driver-
specific members at the end of the struct which will be merged/simplified into the
generic ATAPI handling code in later steps, or removed completely.

Bart:
- move struct ide_atapi_pc outside of #ifdef/#endif CONFIG_IDE_PROC_FS

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