ide-tape: pass packet command in rq->cmd

Make a redundant copy of the packet command bits into rq->cmd. Later, after
all drivers have been converted, it'll be switched to use that in the
common code instead of pc->c. While at it, simplify ide_tape_create_rw_cmd.

There should be no functionality change resulting from this patch.

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