video: Fix cfb_console for 4-bit wide font

The cfb_console can't handle 4-bit wide font properly, since with
4-bit wide font, all 8 bits are drawn. Unbreak the video_drawchars()
function to correctly render 4-bits only on such fonts.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>