Input: stmpe-keypad - remove VLA usage

In preparation to enabling -Wvla, remove VLA and replace it with a
fixed-length array instead.

Fixed as part of the directive to remove all VLAs from the kernel:
https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>