[TCP] MD5: Remove some more unnecessary casting.

while reviewing the tcp_md5-related code further i came across with
another two of these casts which you probably have missed. I don't
actually think that they impose a problem by now, but as you said we
should remove them.

Signed-off-by: Matthias M. Dellweg <2500@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>