cifs: remove redundant assignment to pointer pneg_ctxt

The pointer pneg_ctxt is being initialized with a value that is never
read and it is being updated later with a new value.  The assignment
is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steve French <stfrench@microsoft.com>