auxdisplay: charlcd: fix hex literal ranges for graphics command

The graphics command expects 16 hexadecimal literals, but would allow
characters in range [0-9a-zA-Z] instead of [0-9a-fA-F].

Signed-off-by: Robert Abel <rabel@robertabel.eu>
Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>