ixgb: eliminate checkstack warnings

Really trivial fix, use kmalloc/kfree instead of stack space.
use static const instead of const to further reduce stack usage.

V2: reflect changes suggested by Joe Perches

before:
[jbrandeb@jbrandeb-mobl2 linux-2.6]$ make checkstack|grep '\[ixgb\]'
0x00000fc1 ixgb_set_multi [ixgb]:                       768
0x00001031 ixgb_set_multi [ixgb]:                       768
0x000010f2 ixgb_set_multi [ixgb]:                       768
0x061c ixgb_check_options [ixgb]:                       448
0x09c3 ixgb_check_options [ixgb]:                       448
0x0000649e ixgb_set_ringparam [ixgb]:                   192
0x0000130d ixgb_xmit_frame [ixgb]:                      184
0x000019e0 ixgb_xmit_frame [ixgb]:                      184
0x00002267 ixgb_clean [ixgb]:                           152
0x00002673 ixgb_clean [ixgb]:                           152

after:
0x000064ee ixgb_set_ringparam [ixgb]:                   192
0x0000135d ixgb_xmit_frame [ixgb]:                      184
0x00001a30 ixgb_xmit_frame [ixgb]:                      184
0x000022b7 ixgb_clean [ixgb]:                           152
0x000026c3 ixgb_clean [ixgb]:                           152

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>