cxgb4: remove redundant assignment to hdr_len

Variable hdr_len is being assigned a value that is never read.
The assignment is redundant and hence can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>