mwifiex: declare sta_ptr in smaller scope

sta_ptr is used only in an 'if' branch in this function.
Move it to the smaller scope where it is used.

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