cxgbei: Limit T5 transmit data segments to 15k.

This avoids exceeding a limit in the firmware when using ISO with
jumbo frames.

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

(cherry picked from commit 64f09f2346650f02b6deccbe05bb02b88fce4a5e)