auxdisplay: charlcd: cancel backlight work on registration failure

With CONFIG_CHARLCD_BL_FLASH, charlcd_init() schedules bl_work before
charlcd_register() calls misc_register(). If registration fails, the
caller frees the charlcd object while delayed work still contains its
address.

Add charlcd_deinit() to cancel the delayed work and turn the backlight
off. Use it for both registration rollback and normal unregistration.

Fixes: 39f8ea46724e ("auxdisplay: charlcd: Extract character LCD core from misc/panel")
Cc: stable@vger.kernel.org
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Hongyan Xu <getshell@seu.edu.cn>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>