b43legacy: fix debugfs crash

This patch fixes a crash that happens because b43legacy's
debugfs code expects file->f_op to be a pointer to its own
b43legacy_debugfs_fops struct. This is no longer the case
since commit 9fd4dcec
("debugfs: prevent access to possibly dead file_operations at file open")

Reviewed-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Cc: stable <stable@vger.kernel.org> # 4.7+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>