Add null check for mChannel object in WifiP2Settings

When user switch between settings of hotspot(Enable hotspot),
Internet(Enable Wifi) and Wifi-Direct(establish, disconnect and
forgot P2P connection).

mChannel becomes null during WiFi P2P user scenarios resulting
in Settings(com.android.settings) crash on enable Wifi.

This commit fixes mChannel IllegalArgumentException: Channel
needs to be initialized.

Test: manual test
Bug: 218608961
Change-Id: I72c67bcd1086afe2aa8d2c5521107e2d38e5b0bb