ixgbe: return integer from ixgbe_acquire_msix_vectors

Similar to how ixgbevf handles acquiring MSI-X vectors, we can return an
error code instead of relying on the flag being set. This makes it more
clear that we have failed to setup MSI-X mode, and also will make it
easier to consolidate MSI-X related code all into the single function.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>