cxgb4: memory corruption in debugfs

You can't use kstrtoul() with an int or it causes memory corruption.
Also j should be unsigned or we have underflow bugs.

I considered changing "j" to unsigned long but everything fits in a u32.

Fixes: 8e3d04fd ('cxgb4: Add MPS tracing support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>