leds: qpnp-wled: Fix the string compare operation for dim_mode

The dim_mode value via sysfs carries a new-line character. Use
the correct string to compare.

CRs-Fixed: 1093371
Change-Id: I02f75e32880dc807977db0667062b047237a9b0d
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>