e1000: Never increment the RX undersize count register

In situations where e1000 receives an undersized Ethernet frame,
QEMU increments the emulated "Receive Undersize Count (RUC)"
register when padding the frame.

This is incorrect because this an expected scenario (e.g. with
VLAN tag stripping) and not an error. As such, QEMU should not
increment the emulated RUC.

Fixes: 3b274301 ("e1000: Implementing various counters")

Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Chris Kenna <chris.kenna@oracle.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>