net: sparx5: fix default value of monitor ports

When doing port mirroring, the physical port to send the frame to, is
written to the FRMC_PORT_VAL field of the QFWD_FRAME_COPY_CFG register.
This field is 7 bits wide on sparx5 and 6 bits wide on lan969x, and has
a default value of 65 and 30, respectively (the number of front ports).

On mirror deletion, we set the default value of the monitor port to
65 for this field, in case no more ports exists for the mirror. Needless
to say, this will not fit the 6 bits on lan969x.

Fix this by correctly using the n_ports constant instead.

Fixes: 3f9e4634 ("net: sparx5: use SPX5_CONST for constants which already have a symbol")
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>