diag: Ratelimit error message on failure to write to remote processor

Diag driver is printing an error dmesg on failure to write to diagfwd
bridge. Change pr_err to pr_err_ratelimited to avoid flooding kernel
dmesg.

Change-Id: Iafe6e8270367ac27011b4e3935f67c2f97556e3f
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>