twl4030: fix potential null pointer dereference

The following patch fix a potential null pointer
dereference in twl4030 keypad driver when parts
of keypad platform_data aren't passed down to the
driver. At that point kp->dbg_dev is not set yet.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>