Input: pxa27x - fix keypad KPC macros

We want to mask (key_number - 1), not key_number. The current
implementation works fine for all values but the maximum one,
i.e. 8.

Signed-off-by: Samuel Ortiz <sameo@openedhand.com>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>