diag: Pass correct value when there is a mux write failure

The buffer context passed to the peripheral transport layers if
there are any errors while writing to MUX is incorrect. This patch
fixes this issue.

Change-Id: I6c96b44f3ab7ac0b4ba254a1e04e283360b63ffb
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>