net: ethernet: davicom: fix devicetree irq resource

The dm9000 driver doesn't work in at least one device-tree
configuration, spitting an error message on irq resource :
[    1.062495] dm9000 8000000.ethernet: insufficient resources
[    1.068439] dm9000 8000000.ethernet: not found (-2).
[    1.073451] dm9000: probe of 8000000.ethernet failed with error -2

The reason behind is that the interrupt might be provided by a gpio
controller, not probed when dm9000 is probed, and needing the probe
deferral mechanism to apply.

Currently, the interrupt is directly taken from resources. This patch
changes this to use the more generic platform_get_irq(), which handles
the deferral.

Moreover, since commit Fixes: 7085a740 ("drivers: platform: parse
IRQ flags from resources"), the interrupt trigger flags are honored in
platform_get_irq(), so remove the needless code in dm9000.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Marcel Ziswiler <marcel@ziswiler.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Tested-by: Sergei Ianovich <ynvich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>