cxgb4: clean up a type issue

The tx_desc struct holds 8 __be64 values.  The original code in
ring_tx_db() took a tx_desc pointer then casted it to an int pointer and
then casted it to a u64 pointer.  It was confusing and triggered some
static checker warnings.

I have changed the cxgb_pio_copy() function to only take tx_desc
pointers.  This isn't really a loss of flexibility because anything else
was buggy to begin with.

I also removed the casting on the destination pointer since that was
unnecessary and a bit messy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>