Devices that support jumbo frames will currently default to the
largest possible MTU.  This assumption is valid for virtual adapters
such as virtio-net, where the MTU must have been configured by a
system administrator, but is unsafe in the general case of a physical
adapter.

Default to the standard Ethernet MTU, unless explicitly overridden
either by the driver or via the ${netX/mtu} setting.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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>

Signed-off-by: Michael Brown <mcb30@ipxe.org>