Input: stmpe-keypad - fix valid key line bitmask

The bitmask comment says it will enable GPIO 8-14 and 16-20 for keypad use,
but it actually enables GPIO 8-11 and 13-20 due to a bit error.

Instead of masking of the "hole" at GPIO 12 (which is used for keypad
output 4) mask of the proper "hole" at GPIO 15.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>