drivers: ivshmem: Remove unnecessary BDF lookup code

There was a bunch of BDF lookup code in the ivshmem driver originating
from issues with not being able to have multiple PCIE_BDF_NONE entries
in DTS. Now that devices are always looked up based on VID/DID this
should not be an issue, and hence the additional code is not needed.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>