Input: pxa27x_keypad bug fix for direct_key_mask

When direcct_key_num is 0, the mask should be 0.
When direcct_key_num is 1, the mask should be 0b1.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>