irqchip/econet-en751221: Switch to irq_domain_create_linear()

irq_domain_add_linear() is about to be removed. Switch to
irq_domain_create_linear().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Caleb James DeLisle <cjd@cjdns.fr>
Link: https://lore.kernel.org/all/877c2top39.ffs@tglx