liquidio: VF interrupt initialization cleanup

Set initialization state variable to (reflect interrupt initialization) at
correct time (immediately after having configured interrupts).  This fixes
problem of inconsistent IRQ allocation in case of [obscure] failure when
negotiating with PF driver during init.

Clean-up of interrupt enablement during initialization & avoid potential
race condition with chip-specific code (i.e. perform interrupt control in
main driver module).  Added explanatory comments regarding interrupt
enablement.

Signed-off-by: Rick Farrington <ricardo.farrington@cavium.com>
Signed-off-by: Satanand Burla <satananda.burla@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>