Prevent scanning during DHCP process

Wi-Fi should be in active state during the entire DHCP process, and
shouldn't go to IEEE 802.11 power save mode. If the framework requests
scan during the DHCP process, the Wi-Fi chip has to start scanning
on channels different from the current one, and going to power save
mode is a prerequisite for scan. The result directly impacts user
experience: DHCP process takes longer, and even can fail.

Change-Id: I8171388bb70072e4c42cb3c074dd955da84e494b