fm10k: 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.

Signed-off-by: Scott Peterson <scott.d.peterson@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>