smsc911x.c: encapsulate enable irq calls

We encapsulate enbale irq functionality in a function call.
As on probe the interrupts will be disabled twice, we delete
one.

Signed-off-by: Matthias Brugger <mbrugger@iseebcn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>