igb: remove duplicate code for fallback interrupt initialization

Given a small change to igb_init_interrupt_scheme() the function fits
igb_request_irq() for MSI/legacy interrupts initialization as well, instead of
duplicating most of its code there.

Also adding a missing igb_configure() to igb_request_irq() for MSI fallback
to work properly.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>