e1000: fix ethtool test irq alloc as "probe"

New code added in 2.6.17 caused setup_irq to print a warning when
running ethtool -t eth0 offline.

This test marks the request_irq call made by this test as a "probe"
to see if the interrupt is shared or not.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>