fbcon: fix rotate upside down crash

Fix the rotate_ud() function not to crash in case of a font which has not
a width of multiple by 8: The inner loop of the font pixel copy should not
access a bit outside the font memory area.  Subtract the shift offset from
the font width will prevent this.

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>