iio: backend: fix out-of-bound write

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

iio: backend: fix out-of-bound write

The buffer is set to 80 character. If a caller write more characters,

count is truncated to the max available space in "simple_write_to_buffer".

But afterwards a string terminator is written to the buffer at offset count

without boundary check. The zero termination is written OUT-OF-BOUND.

Add a check that the given buffer is smaller then the buffer to prevent.