it83xx: pwm: fix wrong index of pwm_channels
it83xx: pwm: fix wrong index of pwm_channels

BRANCH=none
BUG=b:194047863
TEST=On spherion, no keyboard backlight blinking.

Signed-off-by: Dino Li <338d523ecf3700bd86eb9c34d220f91911d2a18c@ite.com.tw>
Change-Id: I847291268720bf12ca98b3e38e29a556ec038cd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3043075
Tested-by: Ben Chen <e0a434a5da0616e0b8100476444b79aeee435f0d@quanta.corp-partner.google.com>
Reviewed-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
Reviewed-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>

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