Fix QDeclarativeHapticsEffect::setIntensity()

Previously, the value of the intensity was never modified by
setIntensity().  This commit ensures that the result of the value
comparison is used correctly to determine whether or not to update
the intensity to the new value.

Task-number: QTMOBILITY-2084
Change-Id: I53a0ab027c10b0be70761fbac71c5bd2ad4cec7a