ixgb: fix sparse warnings

Fix sparse warnings in ixgb driver for net-2.6.24.

Added a sparse fix for invalid declaration using non-constant value
in ixgb_set_multi. Added a fix for the module param array index
and allows int params in the array. --Auke

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>