bnxt_en: Improve sysctl handler bnxt_dcb_list_app()

Prefer sbuf_new_for_sysctl() over error-prone manually managed buffer.

No functional change intended.

Reviewed by: markj
Tested by: Daniel Porsch <daniel.porsch@loopia.se>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48496

(cherry picked from commit 91bae242bc54856c560557b5ba245df94d570e95)