Input: tca8418_keypad - use a temporary variable for parent device

Use a temporary variable for our parent device (coming from I2C client
structure); we'll be also using it during conversion to managed resources.

Reviewed-by: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>