i40e/i40evf: Limit DMA sync of RX buffers to actual packet size

On packet RX, we perform a DMA sync for CPU before passing the
packet up.  Here we limit that sync to the actual length of the
incoming packet, rather than always syncing the entire buffer.

Change-ID: I626aaf6c37275a8ce9e81efcaa773f327b331487
Signed-off-by: Scott Peterson <scott.d.peterson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>