fastchg: change fast charge to override the high current GPIO, not the USB cable mode

Signed-off-by: Dan Trifan <jimsth@gmail.com>
fastchg: Remove vestigial fsa9480 "force AC" switch.

Signed-off-by: Dan Trifan <jimsth@gmail.com>
fastchg: Only enable fast charge GPIO as long as we actually have some charging to do.

Signed-off-by: Dan Trifan <jimsth@gmail.com>