V4L/DVB (3704): Fix some errors on bttv_risc_overlay

There are tree mistakes on bttv_risc_overlay.
1) When skip_odd is true, the number of lines for which
instructions are written is (height+1)/2, not height/2.
2) This occurs when clipping: the number of instruction bytes
written can be as much as 8 + 12*nclips, not 8 + 8*nclips, as
currently estimated.
3) Coverity check were wrong with nskips=0, since it means that
it can clipped at most one line.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>