auxdisplay: panel: need to delete scan_timer when misc_register fails in panel_attach

In panel_attach, if misc_register fails, we need to delete scan_timer,
which was setup in keypad_init->init_scan_timer.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>