tg3: Allow single MSI-X vector allocations

This patch changes the code to make it legal to allocate only one MSI-X
vector.  It also fixes a bug where the driver was not checking for error
return codes from pci_enable_msix().

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>