cxgbe(4): Check if the firmware supports 512 SGL per FR MR.

Firmwares >= 1.25.6.0 support 512 SGL entries in a single memory
registration request.

Obtained from: Chelsio Communications
Sponsored by: Chelsio Communications

(cherry picked from commit db15dbf8801120241b7bfb6461341f2cb421ef8e)