mwl8k: Do not call mwl8k_cmd_set_rf_channel unconditionally

Avoid calling mwl8k_cmd_set_rf_channel unconditionally
by checking IEEE80211_CONF_CHANGE_CHANNEL.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>