mwifiex: refactor start_ap handler

Refactor mwifiex_cfg80211_start_ap by creating separate function
to set AP channel and another for configuring other AP parameters
and starting AP.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Qingshui Gao <gaoqs@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>