input_dev may be NULL if mxt_initialize_input_device fails. But pm ops is
still available and suspend/resume assume
input_dev is not NULL. To fix this issue, we add a check if (!input_dev).

Signed-off-by: Pan Xinhui <xinhuix.pan@intel.com>
Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>