TCM buffer length doesn't necessarily equal 8 + ADDITIONAL LENGTH which
might be considered an underflow in case of Data-In size being greater than
8 + ADDITIONAL LENGTH. So truncate buffer length to prevent underflow.

Link: https://lore.kernel.org/r/20210209072202.41154-3-a.miloserdov@yadro.comReviewed-by: NRoman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: NBodo Stroesser <bostroesser@gmail.com>
Signed-off-by: NAleksandr Miloserdov <a.miloserdov@yadro.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>