igb: fix legacy mode irq issue

I booted an igb kernel with the option pci=nomsi and instantly noticed
that interrupts no longer worked on my igb device.  I took a look at the
interrupt initialization and quickly discovered a comment stating:

"DO NOT USE EIAME or IAME in legacy mode"

It seemed a bit odd that bits to enable IAM were being set in legacy
interrupt mode, so I dropped out the following parts and interrupts
began working fine again.

[Updated code flow and a nitpick spelling error --Auke]

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>