ixv(4): Add more robust mailbox API negotiation

The previous update to the driver to 3.2.12-k changed the VF's API version
to 1.2, but did not let the VF fall back to 1.1 or 1.0 versions. So, this
patch tries 1.2 first, then the older versions in succession if that fails.

This should allow the VF driver to negotiate 1.1 and work with older PF
drivers, such as the one used in Amazon's EC2 service.

PR: 220872
Submitted by: Jeb Cramer <jeb.j.cramer@intel.com>
MFC after: 1 week
Sponsored by: Intel Corporation