input: qcom-hv-haptics: Fix a possible NULL pointer dereference

Use Vmax from play effect in haptics_set_gain() only if the effect
is valid. Otherwise, use Vmax from the haptics configuration instead.

Change-Id: I7ec0f8c869096bccb26f7fda0057e3db3a51225d
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>