ide-atapi: compute cmd_len based on device type in ide_transfer_pc

There should be no functionality change resulting from this patch.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
[bart: move cmd_len check closer to ->output_data() call]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>