auxdisplay: charlcd: Move to_priv() to charlcd namespace

In order to be more particular in names, rename to_priv() macro
to charlcd_to_priv().

No functional change intended.

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>