mwifiex: remove redundant code in set channel path

1) Recently we removed set_channel cfg80211 handler. Also, cfg80211
blocks ibss connection requests if ibss network is already started
/joined. Hence the code to restart ibss network in new channel
(mwifiex_drv_change_adhoc_chan() function) becomes redundant.

2) mwifiex_bss_set_channel() function is redundant. It does some
error checking and calculate adhoc start band and adhoc channel.
Cfg80211 already takes care of error checking and provides correct
channel information to the driver. Adhoc start band is already
calculated in mwifiex_set_rf_channel() function.

Other associated code is also removed in this patch.

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>