[ena] Fix spurious uninitialised variable warning on older versions of gcc
Some older versions of gcc (observed with gcc 4.7.2) report a spurious
uninitialised variable warning in ena_get_device_attributes(). Work
around this warning by manually inlining the relevant code (which has
only a single call site).
Reported-by: xbgmsharp <xbgmsharp@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>