input: qcom-hv-haptics: set effect to NULL in constant playing

In constant waveform playing, there is no need to use an effect data
structure to capture the current playing effect, hence set it to NULL.
This also helps the set_gain() callback to use the correct Vmax when
it's called in constant playing case. Also add a debug message to show
the Vmax setting.

Change-Id: I763ec7407b8696300dbe93f5e6935900544e75a8
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>