media: staging/cxd2099: fix debug message severity

Debug messages should go to KERN_DEBUG, thus change the slot_shutdown()
notice from dev_info() to dev_dbg().

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Jasmin Jessich <jasmin@anw.at>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>