video: at91: Adjust vidconsole_position_cursor() to use char pos

video: at91: Adjust vidconsole_position_cursor() to use char pos

At present this function uses pixels but it seems more useful for it to
position in terms of characters on the screen. This also matches the
comment to the function. Update this.

Unfortunately there is one user of this function (at91). Have a crack at
fixing this, since I cannot test it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>