rt2800: debugfs register access: BBP is 256 bytes big

We're already using BBP for values > 128. Make that explicit and allow
debugfs access.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>