When resending an I-frame, ERTM was reusing the control bits from the last
time it was sent, that was causing a corruption in the new control field
due to it dirty fields.

This patches extracts only the SAR bits from the old field and reuse it to
resend the packet, the others bits should be reset and receive the
updated value.
Signed-off-by: NGustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>