serial: mfd: snprintf() returns largish values

snprintf() returns the number of bytes which would have been written so
it can be larger than the size of the buffer.  In this case it's fine,
but people copy and paste this code so I've fixed it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>