rt2x00: debugfs support - allow a register to be empty

Allow a register to be unspecified, therefore not creating its debugfs
file entry.

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>