i40e: Fix legacy interrupt mode in the driver

This patch fixes the driver flow to take into account legacy interrupts.
Over time we added code that assumes MSIX is the only mode that the
driver runs in. It also enables a legacy workaround to trigger SWINT
when the TX ring has non-cache aligned descriptors pending and interrupts
are disabled.

We work with a single vector in MSI mode too, so apply the same
restrictions as Legacy.

Change-ID: I826ddff1f9bd45d2dbe11f56a3ddcef0dbf42563
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>