diag: Make fixes to diag_switch_logging

Diag driver holds on to the socket process task structure even
after signaling the process to exit. This patch clears the internal
handle after signaling.

Change-Id: I642fb595fc2caebc6f2f5419efed4fb560e4e4db
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>