net: sparx5: Fix add vlan when invalid operation

Check if operation is valid before changing any
settings in hardware. Otherwise it results in
changes being made despite it not being a valid
operation.

Fixes: 78eab33b ("net: sparx5: add vlan support")

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>