cw1200: Sanity-check arguments in copy_from_user()

The optional debugfs interface to the vendor's engineering tools wasn't
bounds checking at all, which made it trivial to perform a buffer
overflow if this interface was compiled in and then explicitly enabled
at runtime.

This patch checks both the length supplied as part of the data to ensure
it is sane, and also the amount of data compared to the remaining buffer
space.  If either is too large, fail immediately.

(This bug was spotted by Dan Carpenter <dan.carpenter@oracle.com>)

Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>