[PATCH] e1000: The user-supplied itr setting needs the lower 2 bits masked off

The lower 2 bits of a user-supplied itr setting (via ethtool) need to be
masked off: These lower two bits are used as control bits.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>