net/e1000: advertise offload capabilities for the EM PMD

net/e1000: advertise offload capabilities for the EM PMD

The hardware offload capabilities are not being advertised for the EM PMD.
Because of this, applications that only enable these features if the device
advertises them will never do so.

Normally this is not an issue since normal packet processing should work
even if hardware offload is not available.  But, in older versions of
Virtual Box the e1000 device emulation (Intel PRO/1000 MT Desktop 82540EM)
assumes that it should enable VLAN stripping even if the driver does not
request it.  This means that any ingress packets that have a VLAN tag will
be stripped.  Since the application did not request to enable VLAN
stripping it is not expecting these packets so they are not processed as
VLAN packets.

Regardless of the Virtual Box issue, the driver should be advertising
supported capabilities as is done in other drivers.

Signed-off-by: Allain Legacy <allain.legacy@windriver.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>