Add runtime option for FFC in AC mode

[ThiefMaster]

It would be nice if there was an option (controllable via /sys/kernel/fast_charge/force_fast_charge) to fast-charge in AC mode instead of USB mode. For example, I rarely use the actual USB connection so I'd prefer my PC to not show the "this device would be faster on an USB 3 port" crap whenever I just connect my phone to charge it. Also, when charging it in untrusted places I'd feel better knowing that there will be no USB connectivity at all (I know, with an unlocked bootloader someone could easily backdoor it by simply going through the bootloader/recovery, but that's a different thing)