mwifiex: move station list functions to common code

These functions are now needed by TDLS while managing station list.
Move them from AP related file to utility file.

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