leds: qpnp-wled: Do not program the hybrid dimming register always

Currently hybrid dimming register is programmed always
irrespective of the dim_mode, which is not required. Also make
sure to initialize num_strings variable to '0' when the
led-string-list is empty.

Change-Id: I9a98767d79aabc3826831e378879cdff27f18a6b
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>