net: phy: mv88e61xx: Finish migration of MV88E61XX_FIXED_PORTS

Set the default for MV88E61XX_FIXED_PORTS to 0x0 in Kconfig, and move
the comment from code to the help to explain what this does.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>