debugfs: only accept read attributes for blobs

Blobs can only be read. So, keep only 'read' file attributes because the
others will not work and only confuse users.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210504131350.46586-1-wsa+renesas@sang-engineering.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>