mwifiex: fix potential out-of-boundary access to ibss rate table

smatch found this error:

CHECK   drivers/net/wireless/mwifiex/join.c
  drivers/net/wireless/mwifiex/join.c:1121
  mwifiex_cmd_802_11_ad_hoc_join()
  error: testing array offset 'i' after use.

Cc: <stable@vger.kernel.org> # 3.0+
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>