driver: net: ldpaa: Increase num of buffers for a pool

Management Complex FW 9.0 set the hardware depletion to be 20
buffers in order to support multiple pools in DPNI. This requires
driver to fill the pool with at least 21 to be able to receive
frames. So, Increase number of buffers for a pool.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <york.sun@nxp.com>