bna: use memdup_user to copy userspace buffers

Patch converts kzalloc->copy_from_user sequence to memdup_user. There
is also one useless assignment of NULL to bnad->regdata as it is followed
by assignment of kzalloc output.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>