V4L/DVB (4510): Fix signedness error in drivers/media/video/vivi.c

when checking the -Wextra signedness warnings issued by gcc 4.1
I came across this one:
drivers/media/video/vivi.c:1001: warning: comparison of unsigned expression < 0 is always false
Since videobuf_reqbufs() returns negative values on errors the current
code does no real error checking since gcc removes the comparison.
This patch fixes this issue by making ret a normal, signed integer.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>