if_bnxt: Update Maximum Configurable MTU from 9000 to 9600

Increasing the maximum configurable MTU from 9000 to 9600 to
align with the firmware's capability of handling an MTU up to 9600.

Reviewed by: imp
Approved by: imp
Differential revision: https://reviews.freebsd.org/D42954

(cherry picked from commit 8a3aab53e335dadb3c95c0eb0664143a955f1ab7)