LED On/Off: Allow compiling without CONFIG_CHARGER
LED On/Off: Allow compiling without CONFIG_CHARGER

The header for charge_get_percent() will only define this function if
CONFIG_CHARGER is present.  Make a declaration for complilation to use,
and then allow most of the led_get_state() function to be optimized out
with IS_ENABLED().

BRANCH=None
BUG=b:185508707
TEST=make -j buildall

Signed-off-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>
Change-Id: I3d94f290ac5b9ec82b46b9c0a6fc668b2e8dd91c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2832693
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec