mailbox: Don't check for NULL before ipc_log_string()

The ipc_log_string() function quietly checks for a NULL context so an
explicit check is not needed.

Also fix the string format for QMP_ERROR so that the same string is
generated for both logging methods.

Change-Id: Ic0dedbad69759a9025b41584d13ed6ca982d2015
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>