selftests: bpf: Don't try to read files without read permission

Recently couple of files that are write only were added to netdevsim
debugfs. Don't read these files and avoid error.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>