auxdisplay: Move write_data pointer to hd44780_common

This moves the write_data function pointer from struct charlcd_ops to
struct hd44780_common. This is the function that actually writes the
character to the display. This hd44780 hardware specific function is
used by two drivers at the moment.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>