mwifiex: download cal-data from device-tree to firmware

Currently only conf file based cal-data downloading is supported.
On embedded platforms a better place to store the cal-data is in
device tree. Add cal-data downloading from device tree to firmware.
Both methods can co-exist.

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