cxgbei: Handle target transfers with excess unsolicited data.

The CTL frontend might have provided a buffer that is smaller than the
FirstBurstLength and thus smaller than the amount of unsolicited data
included in the request PDU. Treat these transfers as an empty
transfer.

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

Differential Revision: https://reviews.freebsd.org/D29940