cxgbei: Remove invalid assertion.

A non-placed PDU can be delivered by CPL_RX_ISCSI_CMP in the middle of
a burst of placed PDUs (received via DDP) in which case the rcv_nxt
will not match the start of the non-placed PDU.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications