dsa: mv88e6xxx: Delete ppu timer when removing module

The PPU method of accessing PHYs makes use of a timer. Make sure this
timer is deleted before unloading the driver.

Reported-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>