net: ethernet: ezchip: fix error handling

As documented at drivers/base/platform.c for platform_get_irq:

 * Gets an IRQ for a platform device and prints an error message if finding the
 * IRQ fails. Device drivers should check the return value for errors so as to
 * not pass a negative integer value to the request_irq() APIs.

So, the driver should check that platform_get_irq() return value
is _negative_, not that it's equal to zero, because -ENXIO (return
value from request_irq() if irq was not found) will
pass this check and it leads to passing negative irq to request_irq()

Fixes: 0dd07709

 ("NET: Add ezchip ethernet driver")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>