mwifiex: fix potential buffer overflow in dt configuration

If cfgdata length exceeds the command buffer size we will end up
getting buffer overflow problem. Fix it by checking the buffer
size less the command header length.

Reviewed-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>