cxgb4: kfree mhp after the debug print

In function `c4iw_dealloc_mw`, variable mhp's value is printed after
freed, it is clearer to have the print before the kfree.

Otherwise racing threads could allocate another mhp with the same pointer
value and create confusing tracing.

Signed-off-by: Shaobo He <shaobo@cs.utah.edu>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>