Add dummy ->generic_packet() for devices that don't provide one.  It will
error all commands with "unknown opcode" 0x05/0x20/0x00 sense and return
-EIO.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>