mwifiex: use a function to replace two copies of a code fragment

Instead of having two copies of the code for device tree cfgdata
downloading, add a function to improve the code.

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