rpmsg: 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.

Change-Id: Ic0dedbadbd6f8d3e8e1b76ba5597fa0b6f6ead2a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>