net: dsa: qca8k: fix inband mgmt for big-endian systems

The header and the data of the skb for the inband mgmt requires
to be in little-endian. This is problematic for big-endian system
as the mgmt header is written in the cpu byte order.

Fix this by converting each value for the mgmt header and data to
little-endian, and convert to cpu byte order the mgmt header and
data sent by the switch.

Fixes: 5950c7c0 ("net: dsa: qca8k: add support for mgmt read/write in Ethernet packet")
Tested-by: Pawel Dembicki <paweldembicki@gmail.com>
Tested-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>