V4L/DVB: V4L2: avoid name conflicts in macros

"sd" and "err" are too common names to be used in macros for local variables.
Prefix them with an underscore to avoid name clashing.

[mchehab@redhat.com: whitespace cleanups]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>