net: stmmac: Handle different error codes from platform_get_irq_byname

The following patch moved device tree interrupt resolution into
platform_get_irq_byname:

  ad69674e

 of/irq: do irq resolution in platform_get_irq_byname()

As a result, the function no longer only return -ENXIO on error.
This breaks DT based probing of stmmac, as seen in test runs of
linux-next next-20140526 cubie2-sunxi_defconfig:

  http://lists.linaro.org/pipermail/kernel-build-reports/2014-May/003659.html

This patch makes the stmmac_platform probe function properly handle
error codes, such as returning for deferred probing, and other codes
returned by of_irq_get_by_name.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>