HID: i2c-hid-of: fix NULL-deref on failed power up

A while back the I2C HID implementation was split in an ACPI and OF
part, but the new OF driver never initialises the client pointer which
is dereferenced on power-up failures.

Fixes: b33752c3 ("HID: i2c-hid: Reorganize so ACPI and OF are separate modules")
Cc: stable@vger.kernel.org      # 5.12
Cc: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>