auxdisplay: charlcd: Introduce charlcd_free() helper

The charlcd_free() is a counterpart to charlcd_alloc()
and should be called symmetrically on tear down.

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>