dpaa2-switch: seed the buffer pool after allocating the swp

Any interraction with the buffer pool (seeding a buffer, acquire one) is
made through a software portal (SWP, a DPIO object).
There are circumstances where the dpaa2-switch driver probes on a DPSW
before any DPIO devices have been probed. In this case, seeding of the
buffer pool will lead to a panic since no SWPs are initialized.

To fix this, seed the buffer pool after making sure that the software
portals have been probed and are ready to be used.

Fixes: 0b1b7137 ("staging: dpaa2-switch: handle Rx path on control interface")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>