scsi: sg: Use mult_frac, drop MULDIV macro

The MULDIV macro is essentially a duplicate of the more standard
mult_frac macro. Replace use of MULDIV with mult_frac & drop the
duplication.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>