remoteproc: snprintf() can return more than was printed

snprintf() returns the number of characters which would have been
printed if there were enough space.  For example, on the first print if
we fill up the 28 character string then it would return a number more
than 30.  Use scnprintf() instead because that returns the actual number
of characters printed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>