net: phy: marvell: Fix buffer overrun with stats counters

marvell_get_sset_count() returns how many statistics counters there
are. If the PHY supports fibre, there are 3, otherwise two.

marvell_get_strings() does not make this distinction, and always
returns 3 strings. This then often results in writing past the end
of the buffer for the strings.

Fixes: 2170fef7 ("Marvell phy: add field to get errors from fiber link.")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>