bnxt_en: Fix the description of sysctl knob dev.bnxt.X.dcb.dcbx_cap

While here, update the description of dev.bnxt.X.dcb to more informative
words "Data Center Bridging".

Reviewed by: markj
Fixes: 35b53f8c989f bnxt_en: Add PFC, ETS & App TLVs protocols support
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48993