leds: pwm: Remove a warning on non-DT platforms

This removes a warning on non-DT-enabled platforms:

drivers/leds/leds-pwm.c: In function 'led_pwm_create_of':
drivers/leds/leds-pwm.c:88:22: warning: unused variable 'node'

Really caused by the local variable that is assigned to and then never
used. Just do away with the local var, it's not needed.

Technically this code path can never be entered without DT enabled,
since there's an earlier check about number of children in the calling
function, but the compiler can't see that.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>