staging: frontier: use better DEVICE_ATTR_ macros

Use the DEVICE_ATTR_RO/RW macros to better describe the permissions on
the file, and make them easier to audit.

Cc: David Täht <d@teklibre.com>
Cc: Sara Bird <sara.bird.iar@gmail.com>
Cc: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>