wifihal: fix memory leak in radio_mode module

Radio mode command instance is created when wifihal initialized, but
not deleted when wifihal exit, which causes memory leak.

Fixing this by holding ptr of radio mode instance when initialize,
and delete it when wifihal exit.

CRs-Fixed: 2733746
Change-Id: I06e5ef830dcf4980f527af531d31cd9de5343158