iio:imu: adis16480: show_firmware() buffer too small

Smatch complains that snprintf() returns the number of characters,
not counting the NUL terminator, which *would* have been printed if
there were enough space.  In other words the return value could be more
than sizeof(buf).

In this case, we are printing something like "ff.ff\n" which is at most
6 characters and a NUL so that's not an issue.  I changed snprintf() to
scnprintf() to silence the warning.

But since the buffer doesn't include space for the NUL terminator, we
need to make it bigger or the "\n" will be truncated off.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-By: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>