Currently an invalid port throws a WARN_ON warning however invalid
uninitialized values in reg and cpu_port_index are being used later
on. Fix this by returning -EINVAL for an invalid port value.

Addresses clang-scan warnings:
drivers/net/dsa/qca8k.c:1981:3: warning: 2nd function call argument is an
  uninitialized value [core.CallAndMessage]
drivers/net/dsa/qca8k.c:1999:9: warning: 2nd function call argument is an
  uninitialized value [core.CallAndMessage]

Fixes: 7544b3ff ("net: dsa: qca8k: move pcs configuration")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220224220557.147075-1-colin.i.king@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>