Staging: zcache: signedness bug in tmem_get()

"ret" needs to be signed for the error handling to work properly.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>