diag: Fix improper handling of Diag real time vote IOCTL

Diag driver continues to process data copied from user space even
when it is invalid. Make changes to return from the function when
this happens.

Change-Id: Ieae335c908615dc63b977b7253b0d219e7db600e
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>