if_bnxt: Incorrect Labeling of Supported Medium in ifconfig -m <inf> Command

The 1G speed on DAC medium is incorrectly labeled as 1000baseT, it
should be 1000baseCX. Updated the label accordingly.

Reviewed by: imp
Approved by: imp
Differential revision: https://reviews.freebsd.org/D42952

(cherry picked from commit c5108e37fbf93a538c76325766b8a3d76a72f119)