bna: integer overflow bug in debugfs

We could allocate less memory than intended because we do:

	bnad->regdata = kzalloc(len << 2, GFP_KERNEL);

The shift can overflow leading to a crash.  This is debugfs code so the
impact is very small.

Fixes: 7afc5dbd ("bna: Add debugfs interface.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rasesh Mody <rasesh.mody@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>