mwifiex: remove 11N_CFG command response handler

HostCmd_CMD_11N_CFG is never called with GET action. So the code in
mwifiex_ret_11n_cfg() becomes redundant.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>