Input: twl4030_keypad - fix potential NULL dereference in twl4030_kp_probe()

We should first check whether platform data is NULL or not, before
dereferencing it to get the keymap.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>