Input: atkbd - switch to use scnprintf() to suppress truncation warning

Switch the driver to use scnprintf() to avoid warnings about potential
truncation of "phys" field which we can tolerate.

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>