net: macb: Add support for partial store and forward

- Validate FCS in receive interrupt handler if Rx checksum offloading
  is disabled
- Get rx-watermark value from DT

Signed-off-by: Maulik Jodhani <maulik.jodhani@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>