staging: vc04_services: fix debugfs write functions

write functions need to return ssize_t, not int, so fix the functions up
for the correct prototype.

Cc: Daniel Stone <daniels@collabora.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Pranith Kumar <bobby.prani@gmail.com>
Cc: popcornmix <popcornmix@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>