diag: dci: Check for valid peripheral before writing over to it

Currently while writing data to the peripherals over dci channel,
invalid channel access may happen. This patch add an error check
for such case.

Change-Id: I6a7bdb9c75c4863394dcab12b41387b081c2329f
Signed-off-by: Katish Paran <kparan@codeaurora.org>