stmmac: dwmac-sunxi: Call exit cleanup function in probe error path

dwmac-sunxi has 2 callbacks that were called from stmmac_platform as
part of the probe and remove sequences.

Ater the conversion of dwmac-sunxi into a standalone platform driver,
the .init function is called before calling into the stmmac driver
core, but .exit is not called to clean up if stmmac returns an error.

This patch fixes the probe error path. This properly cleans up and
releases resources when the driver core fails to probe.

Cc: Joachim Eastwood <manabian@gmail.com>
Fixes: 9a9e9a1e ("stmmac: dwmac-sunxi: turn setup callback into a
		      probe function")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>