IB/usnic: check for allocation failure

usnic_uiom_get_dev_list() can return ERR_PTR(-ENOMEM) so we should check
for that.

Fixes: e3cf00d0 ("IB/usnic: Add Cisco VIC low-level hardware driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>