net: dsa: mv88e6xxx: rename mv88e6xxx_g2_scratch_gpio_set_smi

The name mv88e6xxx_g2_scratch_gpio_set_smi is a bit ambiguous as it appears
to only be applicable to the 6390 family, so lets rename it to
mv88e6390_g2_scratch_gpio_set_smi to make it more obvious.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>