auxdisplay: panel: Fix an API misuse in panel.c

Variable allocated by charlcd_alloc() should be released
by charlcd_free(). The following patch changed kfree() to
charlcd_free() to fix an API misuse.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: 718e05ed ("auxdisplay: Introduce hd44780_common.[ch]")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>