RDMA/usnic: Fix remove address space warning

Sparse tool complains with the following error:
drivers/infiniband/hw/usnic/usnic_ib_main.c:445:16: warning: cast removes
	address space of expression

Fix it by doing casting on correct field and convert function helper which
sets ifaddr to be void, because there are no users who are interested in
returned value.

Fixes: c7845bca ("IB/usnic: Add UDP support in u*verbs.c, u*main.c and u*util.h")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>