genet: Fix potential crash during attach

As pointed out in the bug, the genet driver (RPi4 Ethernet) was
attaching the interrupts before the data structures were fully
initialized, causing a crash if an interrupt came in during the
attach. Fix by reordering code blocks.

PR: 256334
Reported by: < ghuckriede at blackberry.com >
Reviewed by: < ghuckriede at blackberry.com > (informally)
MFC after: 3 days