selftests: nci: Add the flags parameter for the send_cmd_mt_nla

To reuse the send_cmd_mt_nla for NLM_F_REQUEST and NLM_F_DUMP flag,
add the flags parameter to the function.

Signed-off-by: Bongsu Jeon <bongsu.jeon@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>