auxdisplay: hd44780: Convert to use charlcd_free()

Convert to use charlcd_free() instead of kfree() for sake of type check.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>