wimax: fix typo in kernel-doc for debugfs_dentry in struct wimax_dev

The kernel-doc was referring to member @debufs_dentry instead of
@debugfs_dentry.

Reported by Randy Dunlap http://marc.info/?l=linux-netdev&m=123147942302885&w=2

As well, escape the colon in the field's text description, as it is
causing the generated text to be erraticly broken up (with paragraphs
moved down). Could not find a reason why it is happening so, even when
other field descriptions use colons and work as expected.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>