cxgbei: Round up the maximum PDU data length by the MSS for TXDATAPLEN_MAX.

Recent firmware versions round down the value passed here by the MSS
and subsequently mishandle transmitted PDUs larger than the rounded
down value.

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

(cherry picked from commit d0d631d5f4437223664f7bbdfdb421ec05cf9657)