ptp: ixp46x: remove NO_IRQ handling

gpio_to_irq does not return NO_IRQ but instead returns a negative
error code on failure. Returning NO_IRQ from the function has no
negative effects as we only compare the result to the expected
interrupt number, but it's better to return a proper failure
code for consistency, and we should remove NO_IRQ from the kernel
entirely.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>