IB/mthca: Enable MSI-X by default

Recover from MSI-X errors by automatically falling back on regular
interrupt, instead of asking the user to do this manually.  This makes
it possible to enable MSI-X by default, and will make it possible to
get rid of the msi_x module option in the future.

Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>