I-Frame which is going to be resend already has FCS field added and set
(if it was required). Adding additional FCS field calculated from data +
old FCS in resend function is incorrect. This patch fix that.

Issue has been found during PTS testing.
Signed-off-by: NLukasz Rymanowski <lukasz.rymanowski@tieto.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>