tg3: Fix single-vector MSI-X code

Kdump kernels leave MSI-X interrupts (as setup by the crashed kernel)
enabled.  However, kdump only enables one CPU in the new environment,
thus causing tg3 to abort MSI-X setup.  When the driver attempts to
enable INTA or MSI interrupt modes on a kdump kernel, interrupt
delivery fails.

This patch attempts to workaround the problem by forcing the driver
to enable a single MSI-X interrupt.  In such a configuration, the
device's multivector interrupt mode must be disabled.

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