cxgb{3,4}*: improve Kconfig dependencies

- Remove the dependency of cxgb4 and cxgb4vf on INET.  cxgb3 really
  depends on INET, keep it but add it directly to the driver's Kconfig
  entry.
- Make the iSCSI drivers cxgb3i and cxgb4i available in the SCSI menu
  without requiring any options in the net driver menu to be enabled
  first.  Add needed selects so the iSCSI drivers can build their
  corresponding net drivers.
- Remove CHELSIO_T*_DEPENDS.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Acked-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>