leds: qti-flash: Allow symmetrical switch on with no channels configured

Currently, with LED symmetry enabled, turning on a switch without
configuring current in a channel first results in errors being seen.

Without LED symmetry enabled, the same action results only in
qti_flash_led_strobe() being called with the value argument set to zero.
Bring about the same behaviour for LED symmetry as well.

Change-Id: I2d6301ec3cc19d89acbd0545509485866ffa9085
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>