stmmac: dwmac-rk: turn exit into standard driver remove callback

Convert the exit hook into a standard driver remove function as
the hook doesn't really buy us anything extra.

Eventually the exit hook will be deprecated in favor of the driver
remove function.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>