net: dsa: microchip: ksz9477: fix fdb_dump last invalid entry

In the ksz9477_fdb_dump function it reads the ALU control register and
exit from the timeout loop if there is valid entry or search is
complete. After exiting the loop, it reads the alu entry and report to
the user space irrespective of entry is valid. It works till the valid
entry. If the loop exited when search is complete, it reads the alu
table. The table returns all ones and it is reported to user space. So
bridge fdb show gives ff:ff:ff:ff:ff:ff as last entry for every port.
To fix it, after exiting the loop the entry is reported only if it is
valid one.

Fixes: b987e98e

 ("dsa: add DSA switch driver for Microchip KSZ9477")
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220816105516.18350-1-arun.ramadoss@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>