cadence_gem: Fix Rx buffer size field mask

cadence_gem: Fix Rx buffer size field mask

This patch corrects the Rx buffer size field mask to mask bits 23 to 16
to match Xilinx UG585 documentation.

Signed-off-by: Sai Pavan Boddu <saipava@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>