Input: matrix_keypad - remove redundant platform_set_drvdata()

Commit 0998d063

 (device-core: Ensure drvdata = NULL when no
driver is bound) removes the need to set driver data field to
NULL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>