cxgbei: Adjust the calculation for the maximum ISO payload.

Round down the maximim ISO payload by the current MSS. Otherwise the
round up by MSS when calculating the 16-bit maximum payload len passed
along in the FLOWC work request can overflow.

Discussed with: np
Sponsored by: Chelsio Communications

(cherry picked from commit 8020c05683f266c5513cfe8ad3e58e7e06f85ffa)