bnxt: add guards around struct_group macros

The header file might be included after linux/stddef.h or others are
included and the macros would be re-defined.

Sponsored by: The FreeBSD Foundation
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50156

(cherry picked from commit 152e6197615570e7a2f5f1c6c2ed00ecee9dd10c)