Flapjack: Turn off LED when brightness == 0
Flapjack: Turn off LED when brightness == 0

Currently, when the brightness passed to led_set_brightness is zero,
the current is set to 4mA and the pwm duty is set to the brightness.
Since duty == 0 means 1/32 (instead of 0/32), this doesn't turn off
the LED.

This patch makes the current explicitly set to 0mA if the brightness
is zero.

Signed-off-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

BUG=b/131205169
BRANCH=none
TEST=buildall

Change-Id: I7f581349713d30f10acb3797ab08b15aa2d50f00
Reviewed-on: https://chromium-review.googlesource.com/1584351
Commit-Ready: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Tested-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-by: YH Lin <a608809fda0b3a5e5ffc78f5b79106a21e96e6dd@chromium.org>

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