mwifiex: remove redundant band config code

struct mwifiex_ds_band_cfg and mwifiex_set_radio_band_cfg() routine
are unnecessary. It can be done with simple equivalant code.

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