ipl: Limit transferred bytes per PDC IODC call to 4 MB

Limit size of bytes transferred per PDC call to 4 MB, independend
if we read in block mode or not.

I had issues loading the inited from boot disc on the C8000, while it
succeeded on a C3700. The initial bytes of the initrd seemed to be
random data. The transfer size was 16 MB.  It seems the PDC call
returned success, although it did not read the block from disk. Maybe it
just read 16 MB modulo 4 MB -> 0 bytes.

I used a ZuluSCSI drive to emulate the CD. Maybe that was the reason,
but in any way avoid similiar painful debug sessions in the future by
limiting to 4 MB. Since this is a boot loader, performance shouldn't be
that critical.

Signed-off-by: Helge Deller <deller@gmx.de>