leds:lp55xx: Correct Kconfig dependency for f/w user helper

The commit [b6789320: leds:lp55xx: fix firmware loading error]
tries to address the firmware file handling with user helper, but it
sets a wrong Kconfig CONFIG_FW_LOADER_USER_HELPER_FALLBACK.  Since the
wrong option was enabled, the system got a regression -- it suffers
from the unexpected long delays for non-present firmware files.

This patch corrects the Kconfig dependency to the right one,
CONFIG_FW_LOADER_USER_HELPER.  This doesn't change the fallback
behavior but only enables UMH when needed.

Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=944661
Fixes: b6789320

 ('leds:lp55xx: fix firmware loading error')
Cc: <stable@vger.kernel.org> # v4.2+
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>