Input: atkbd - fix overrun in atkbd_set_repeat_rate()

This was introduced in commit 3d0f0fa0:
bounds checking is performed against period[32] while indexing delay[4].

Spotted by Coverity, CID 1376.

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>