qlcnic: potential dereference null pointer of rx_queue->page_ring

The return value of kcalloc() needs to be checked.
To avoid dereference of null pointer in case of the failure of alloc.
Therefore, it might be better to change the return type of
qlcnic_sriov_alloc_vlans() and return -ENOMEM when alloc fails and
return 0 the others.
Also, qlcnic_sriov_set_guest_vlan_mode() and __qlcnic_pci_sriov_enable()
should deal with the return value of qlcnic_sriov_alloc_vlans().

Fixes: 154d0c81

 ("qlcnic: VLAN enhancement for 84XX adapters")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>