mips: kernel: use DEFINE_DEBUGFS_ATTRIBUTE with debugfs_create_file_unsafe()

debugfs_create_file_unsafe does not protect the fops handed to it
against file removal. DEFINE_DEBUGFS_ATTRIBUTE makes the fops aware of
the file lifetime and thus protects it against removal.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>