iio: fix potential out-of-bound write

In the Linux kernel, the following vulnerability has been resolved:

iio: fix potential out-of-bound write

The buffer is set to 20 characters. If a caller write more characters, count is truncated to the max available space in "simple_write_to_buffer". To protect from OoB access, check that the input size fit into buffer and add a zero terminator after copy to the end of the copied data.