leds: qpnp-vibrator: Do not reset the play duration to '0'

Currently vibrator play duration is reset to '0' during the
'activate' operation. Due to this the vibrator does not play
for the subsequent 'activate' operations. Fix it.

CRs-Fixed: 2357595
Change-Id: I96a4f7616d21662affc76bb3e1a4955040f8813d
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>