Clear CPL_GET_TCB_RPL handler on module unload.

This fixes a panic when unloading and reloading t4_tom.ko since the
old pointer is still stored when t4_tom_load tries to set it.

Reviewed by:	np
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D24358

sys/dev/cxgbe/tom/t4_tom.c

@@ -1895,6 +1895,7 @@ t4_tom_mod_unload(void)
 	t4_uninit_listen_cpl_handlers();
 	t4_uninit_cpl_io_handlers();
 	t4_register_shared_cpl_handler(CPL_L2T_WRITE_RPL, NULL, CPL_COOKIE_TOM);
+	t4_register_cpl_handler(CPL_GET_TCB_RPL, NULL);
 
 	return (0);
 }