console: Correct truetype spacing error

The putc_xy() method is supposed to return the amount of space used. The
existing implementation erroneously adds the previous sub-pixel position
to the returned value. This spaces out the characters very slightly more
than it should. It is seldom noticeable but it does make accurate
measurement of the text impossible.

Fix this minor but long-standing bug.

Fixes: a29b0120 ("video: Add a console driver that uses TrueType fonts")

Signed-off-by: Simon Glass <sjg@chromium.org>