The gianfar driver has recently been enabled on arm64 but fails to build
since it check the return value of platform_get_irq() against NO_IRQ. Fix
this by instead checking for a negative error code.

Even on ARM where this code was previously being built this check was
incorrect since platform_get_irq() returns a negative error code which
may not be exactly the (unsigned int)(-1) that NO_IRQ is defined to be.
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>