[SCSI] mpt fusion: Change call back indices to u8 from int

The call back index requires only u8 but in lot of places it is
referred as int, now everywhere the call back index variables are
declared as u8 with uniform name cb_idx

signed-off-by: Sathya Prakash <sathya.prakash@lsi.com>
Acked-by: Eric Moore <Eric.Moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>