mwifiex: add support to use basic rates in ibss join request

In mwifiex_set_rf_channel() ibss specific flags were unnecessarily
getting modified for infra and AP mode. This patch removes
mwifiex_set_rf_channel() function and adds equivalant code in infra,
ibss and AP path.

For ibss, now we are chosing band based on channel type and basic
rates provided in ibss join request. We can start ibss network in
A only, B only, G only, BG, BGN, AN mode.

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>