vibrator: qpnp-haptic: cleanup string for change in play_mode

Clear the unnecessary characters such as newline to allow user
to configure play_mode for other inputs.

CRs-fixed: 773098
Change-Id: I68e460164af0d7de348cca2ccb4a45aa30081c4b
Signed-off-by: Alex Sarraf <asarraf@codeaurora.org>