net/cxgb4: use remove handler as shutdown handler

Without a shutdown handler, T4 cards behave very badly after a kexec.
Some firmware calls return errors indicating allocation failures, for
example. This is probably because thouse resources were not released by
a BYE message to the firmware, for example.

Using the remove handler guarantees we will use a well tested path.

With this patch I applied, I managed to use kexec multiple times and
probe and iSCSI login worked every time.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>