vgacon: vgacon_scrolldelta simplification

There's no point in checking diff == c->vc_rows, because it can be true
only when count == 0, but we already checked that.  Additionally move
variables used only in one block to this block.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Antonino Daplas <adaplas@gmail.com>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>